fix: unify fallthrough annotations

Compiling with -Wimplicit-fallthrough=3 (enabled by -Wextra) produces
many warnings about fallthrough comments either missing or being wrong.
Unify the comments so we comply with -Wextra.

Note that Coverity recommends against using the __attribute__ directive.
Also, zlib does not build with a higher value of -Wimplicit-fallthrough.
Finally, compilers strip comments before expanding macros. As such,
checkpatch's fallthrough annotation (or higher levels of the flag) isn't
really possible.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I060cf4f8dc04c02cbb45cf4ceb69569a8369ccee
diff --git a/drivers/imx/usdhc/imx_usdhc.c b/drivers/imx/usdhc/imx_usdhc.c
index 07f55b7..49dfc07 100644
--- a/drivers/imx/usdhc/imx_usdhc.c
+++ b/drivers/imx/usdhc/imx_usdhc.c
@@ -136,7 +136,8 @@
 		break;
 	case MMC_CMD(18):
 		multiple = 1;
-		/* fall thru for read op */
+		/* for read op */
+		/* fallthrough */
 	case MMC_CMD(17):
 	case MMC_CMD(8):
 		mixctl |= MIXCTRL_DTDSEL;
@@ -144,7 +145,8 @@
 		break;
 	case MMC_CMD(25):
 		multiple = 1;
-		/* fall thru for data op flag */
+		/* for data op flag */
+		/* fallthrough */
 	case MMC_CMD(24):
 		data = 1;
 		break;