drivers: fix switch statements to comply with MISRA rules

Ensure (where possible) that switch statements in drivers comply with
MISRA rules 16.1 - 16.7.

Change-Id: I7a91e04b02af80fbc4673a52293386c0f81a0f7a
Signed-off-by: Jonathan Wright <jonathan.wright@arm.com>
diff --git a/drivers/synopsys/emmc/dw_mmc.c b/drivers/synopsys/emmc/dw_mmc.c
index 701e6d5..156c2b4 100644
--- a/drivers/synopsys/emmc/dw_mmc.c
+++ b/drivers/synopsys/emmc/dw_mmc.c
@@ -321,6 +321,7 @@
 		break;
 	default:
 		assert(0);
+		break;
 	}
 	dw_set_clk(clk);
 	return 0;