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/lib/dp_aux.c

@@ -20,13 +20,13 @@ enum i2c_over_aux dp_get_aux_cmd(enum aux_request request, uint32_t remaining_af
 	case I2C_RAW_WRITE_AND_STOP:
 		if (!remaining_after_this)
 			return I2C_OVER_AUX_WRITE_MOT_0;
-		/* fallthrough */
+		__fallthrough;
 	case I2C_RAW_WRITE:
 		return I2C_OVER_AUX_WRITE_MOT_1;
 	case I2C_RAW_READ_AND_STOP:
 		if (!remaining_after_this)
 			return I2C_OVER_AUX_READ_MOT_0;
-		/* fallthrough */
+		__fallthrough;
 	case I2C_RAW_READ:
 		return I2C_OVER_AUX_READ_MOT_1;
 	case DPCD_WRITE: