mmc: sdhci-xenon: mark expected switch fall-through

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

Warning level 2 was used: -Wimplicit-fallthrough=2
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/sdhci-xenon-phy.c

@@ -526,6 +526,7 @@ static bool xenon_emmc_phy_slow_mode(struct sdhci_host *host,
 			ret = true;
 			break;
 		}
+		/* else: fall through */
 	default:
 		reg &= ~XENON_TIMING_ADJUST_SLOW_MODE;
 		ret = false;