rtw88: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a
warning by replacing a /* fall through */ comment with the new
pseudo-keyword macro fallthrough; instead of letting the code fall
through to the next case.

Notice that Clang doesn't recognize /* fall through */ comments as
implicit fall-through markings.

Link: https://github.com/KSPP/linux/issues/115Signed-off-by: NGustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/967a171da3db43e4cdf38104876b4ec1cde46359.1605896060.git.gustavoars@kernel.org

drivers/net/wireless/realtek/rtw88/fw.c

@@ -1477,7 +1477,7 @@ static bool rtw_fw_dump_check_size(struct rtw_dev *rtwdev,
 	case RTW_FW_FIFO_SEL_RX:
 		if ((start_addr + size) > rtwdev->chip->fw_fifo_addr[sel])
 			return false;
-		/*fall through*/
+		fallthrough;
 	default:
 		return true;
 	}