ath9k: ar5008_phy: mark expected switch fall-through

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

Addresses-Coverity-ID: 1056532 ("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/ath/ath9k/ar5008_phy.c

@@ -586,7 +586,7 @@ static void ar5008_hw_init_chain_masks(struct ath_hw *ah)
 			REG_WRITE(ah, AR_PHY_CAL_CHAINMASK, 0x7);
 			break;
 		}
-		/* else: fall through */
+		/* fall through */
 	case 0x1:
 	case 0x2:
 	case 0x7: