Use the fallthrough statement in switch loops

Clang does not seem to work with 'fall through' in comments.

Change-Id: Idcbe373be33ef7247548f856bfaba7ceb7f749b5
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51498
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

src/console/vtxprintf.c

@@ -244,7 +244,7 @@ repeat:
 
 		case 'X':
 			flags |= LARGE;
-			/* fall through */
+			__fallthrough;
 		case 'x':
 			base = 16;
 			break;
@@ -252,6 +252,7 @@ repeat:
 		case 'd':
 		case 'i':
 			flags |= SIGN;
+			__fallthrough;
 		case 'u':
 			break;