Don't enter Sink Standby if we don't have to

According to the USB Power Delivery Specification Revision 3.0, Version
1.1, section 7.2.3.1, we're not required to enter Sink Standby when
changing the negotiated voltage on a single PPS APDO.  Now we don't.
This is done in the PE, so the DPM doesn't have to lift a finger to
avoid the unnecessary standby.

lib/include/pdb_pe.h

     int8_t _old_tcc_match;
     /* The index of the first PPS APDO */
     uint8_t _pps_index;
+    /* The index of the just-requested PPS APDO */
+    uint8_t _last_pps;
     /* Virtual timer for SinkPPSPeriodicTimer */
     virtual_timer_t _sink_pps_periodic_timer;
     /* Queue for the PE mailbox */

lib/src/policy_engine.c

 
 static enum policy_engine_state pe_sink_eval_cap(struct pdb_config *cfg)
 {
-    /* If we have a message, remember the index of the first PPS APDO so we can
-     * check if the request is for a PPS APDO in PE_SNK_Select_Cap. */
+    /* If we have a Source_Capabilities message, remember the index of the
+     * first PPS APDO so we can check if the request is for a PPS APDO in
+     * PE_SNK_Select_Cap. */
     if (cfg->pe._message != NULL) {
         /* Start by assuming we won't find a PPS APDO (set the index greater
          * than the maximum possible) */
                 break;
             }
         }
+        /* New capabilities also means we can't be making a request from the
+         * same PPS APDO */
+        cfg->pe._last_pps = 8;
     }
     /* Get a message object for the request if we don't have one already */
     if (cfg->pe._last_dpm_request == NULL) {
         cfg->pe._last_dpm_request = chPoolAlloc(&pdb_msg_pool);
+    } else {
+        /* Remember the last PDO we requested if it was a PPS APDO */
+        if (PD_RDO_OBJPOS_GET(cfg->pe._last_dpm_request) >= cfg->pe._pps_index) {
+            cfg->pe._last_pps = PD_RDO_OBJPOS_GET(cfg->pe._last_dpm_request);
+        /* Otherwise, forget any PPS APDO we had requested */
+        } else {
+            cfg->pe._last_pps = 8;
+        }
     }
     /* Ask the DPM what to request */
     cfg->dpm.evaluate_capability(cfg, cfg->pe._message,
         if (PD_MSGTYPE_GET(cfg->pe._message) == PD_MSGTYPE_ACCEPT
                 && PD_NUMOBJ_GET(cfg->pe._message) == 0) {
             /* Transition to Sink Standby if necessary */
-            cfg->dpm.transition_standby(cfg);
+            if (PD_RDO_OBJPOS_GET(cfg->pe._last_dpm_request) != cfg->pe._last_pps) {
+                cfg->dpm.transition_standby(cfg);
+            }
 
             cfg->pe._min_power = false;
 
     cfg->pe._old_tcc_match = -1;
     /* Initialize the pps_index */
     cfg->pe._pps_index = 8;
+    /* Initialize the last_pps */
+    cfg->pe._last_pps = 8;
     /* Initialize the PD message header template */
     cfg->pe.hdr_template = PD_DATAROLE_UFP | PD_POWERROLE_SINK;