device/oprom/yabel/io: use __fallthrough instead of comment

Unlike gcc, a clang build will fail when only a comment is used to
indicate that the fallthough is intended. To fix the clang build, use
__fallthrough instead. This will fix the build errors introduced by
commit f45c7671d9 ("Set x86_64 as supported architecture for clang")
that enabled clang builds for a case that uses yabel to run the VBIOS.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I4ed337025adeb833f352d198fc0f13b5e1c209c4
Reviewed-on: https://review.coreboot.org/c/coreboot/+/70889
Reviewed-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>
Reviewed-by: Martin Roth <martin.roth@amd.corp-partner.google.com>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-by: Fred Reitberger <reitbergerfred@gmail.com>
Reviewed-by: Felix Singer <felixsinger@posteo.net>

src/device/oprom/yabel/io.c

@@ -184,7 +184,7 @@ my_inb(X86EMU_pioAddr addr)
 				X86EMU_trace_on();
 			}
 			M.x86.debug &= ~DEBUG_DECODE_NOPRINT_F;
-			// fall-through
+			__fallthrough;
 		default:
 			DEBUG_PRINTF_IO
 			    ("%s(%04x) reading from bios_device.io_buffer\n",