Maintain HardResetCounter

It's simpler than I realized.  All that's left to do here is the
fallback mechanism for when the Sink is configured for 5 V.  This means
that yet again, a TODO was replaced with another, less critical TODO.

src/pd.h

 #define PD_T_TYPEC_SINK_WAIT_CAP MS2ST(465)
 
 
+/*
+ * Counter maximums
+ */
+#define PD_N_HARD_RESET_COUNT 2
+
+
 /*
  * Unit conversions
  *
 #define PD_PDI_CA(pdv) ((pdv) % 100)
 
 
-
 #endif /* PDB_PD_H */

src/policy_engine.c

 static bool capability_match = false;
 /* Whether or not we have an explicit contract */
 static bool explicit_contract = false;
+/* Keep track of how many hard resets we've sent */
+static int hard_reset_counter = 0;
 /* Policy Engine thread mailbox */
 static msg_t pdb_pe_mailbox_queue[PDB_MSG_POOL_SIZE];
 mailbox_t pdb_pe_mailbox;
 
 static enum policy_engine_state pe_sink_hard_reset(void)
 {
+    /* If we've already sent the maximum number of hard resets, give up */
+    if (hard_reset_counter > PD_N_HARD_RESET_COUNT) {
+        pdb_dpm_output_off();
+        /* TODO: Fall back to USB BC or Type-C power if configured for 5 V. */
+        for (;;)
+            chThdSleepMilliseconds(1000);
+    }
+
     /* Generate a hard reset signal */
     chEvtSignal(pdb_hardrst_thread, PDB_EVT_HARDRST_RESET);
     chEvtWaitAny(PDB_EVT_PE_HARD_SENT);
 
-    /* TODO: maintain HardResetCounter */
+    /* Increment HardResetCounter */
+    hard_reset_counter++;
 
     return PESinkTransitionDefault;
 }