prism54: isl_38xx: Mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 114944 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/intersil/prism54/isl_38xx.c

@@ -235,6 +235,7 @@ isl38xx_in_queue(isl38xx_control_block *cb, int queue)
 		/* send queues */
 	case ISL38XX_CB_TX_MGMTQ:
 		BUG_ON(delta > ISL38XX_CB_MGMT_QSIZE);
+		/* fall through */
 
 	case ISL38XX_CB_TX_DATA_LQ:
 	case ISL38XX_CB_TX_DATA_HQ: