bwi: Mark write-only variable as __unused

We read TX_STATUS1 to acknowledge a TX interrupt. We don't use this
value for anything, so mark it as unused. We may be able to eliminate
this read, but since this hardware is poorly documented and difficult to
test, I'm leaving the read in place.

Sponsored by:		Netflix

sys/dev/bwi/if_bwi.c

@@ -3394,7 +3394,7 @@ bwi_txeof(struct bwi_softc *sc)
 {
 
 	for (;;) {
-		uint32_t tx_status0, tx_status1;
+		uint32_t tx_status0, tx_status1 __unused;
 		uint16_t tx_id;
 		int data_txcnt;