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/drivers/ipmi/ipmi_kcs_ops.c

@@ -247,7 +247,7 @@ ipmi_write_acpi_tables(const struct device *dev, unsigned long current,
 		break;
 	default:
 		printk(BIOS_ERR, "IPMI: Unsupported register spacing for SPMI\n");
-		/* fall through */
+		__fallthrough;
 	case 1:
 		addr.bit_offset = 8;
 		break;
@@ -369,7 +369,7 @@ static int ipmi_smbios_data(struct device *dev, int *handle,
 		break;
 	default:
 		printk(BIOS_ERR, "IPMI: Unsupported register spacing for SMBIOS\n");
-		/* fall through */
+		__fallthrough;
 	case 1:
 		register_spacing = 0 << 6;
 		break;