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>
diff --git a/src/mainboard/google/poppy/variants/nami/mainboard.c b/src/mainboard/google/poppy/variants/nami/mainboard.c
index a29b9f3..629469d 100644
--- a/src/mainboard/google/poppy/variants/nami/mainboard.c
+++ b/src/mainboard/google/poppy/variants/nami/mainboard.c
@@ -241,7 +241,7 @@
 	case SKU_6_SYNDRA:
 	case SKU_7_SYNDRA:
 		pl2_id = PL2_ID_SONA_SYNDRA;
-		/* fallthrough */
+		__fallthrough;
 	case SKU_0_VAYNE:
 	case SKU_1_VAYNE:
 	case SKU_2_VAYNE: