Request less power when output is disabled

Since the spec says we Shall request no more power than we actually
need, we were violating the spec whenever the output was disabled in
Setup mode.  This commit fixes that, requesting low current at 5 V when
the output is disabled.

We do play a little fast and loose with the MAX_CURRENT field, but
that's hard to avoid in a device like this.  Well, we could always set
it to 5 A in Setup mode to be safe, but that's not necessarily right
either.  At least it's still honest in Sink mode, which I consider more
important anyway.

src/device_policy_manager.c

     dpm_unconstrained_power = capabilities->obj[0] & PD_PDO_SRC_FIXED_UNCONSTRAINED;
 
     /* Make sure we have configuration */
-    if (cfg != NULL) {
+    if (cfg != NULL && pdb_dpm_output_enabled) {
         /* Look at the PDOs to see if one matches our desires */
         for (uint8_t i = 0; i < numobj; i++) {
             /* Fixed Supply PDOs come first, so when we see a PDO that isn't a
         PD_SPECREV_2_0 | PD_POWERROLE_SINK | PD_NUMOBJ(1);
     request->obj[0] = PD_RDO_FV_MAX_CURRENT_SET(DPM_MIN_CURRENT)
         | PD_RDO_FV_CURRENT_SET(DPM_MIN_CURRENT)
-        | PD_RDO_NO_USB_SUSPEND | PD_RDO_CAP_MISMATCH
+        | PD_RDO_NO_USB_SUSPEND
         | PD_RDO_OBJPOS_SET(1);
+    /* If the output is enabled and we got here, it must be a capability
+     * mismatch. */
+    if (pdb_dpm_output_enabled) {
+        request->obj[0] |= PD_RDO_CAP_MISMATCH;
+    }
+    /* If we can do USB communications, tell the power supply */
     if (pdb_dpm_usb_comms) {
         request->obj[0] |= PD_RDO_USB_COMMS;
     }
     /* Update requested voltage */
     dpm_requested_voltage = PD_MV2PDV(5000);
 
-    return false;
+    /* At this point, we have a capability match iff the output is disabled */
+    return !pdb_dpm_output_enabled;
 }
 
 void pdb_dpm_get_sink_capability(union pd_msg *cap)