treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
diff --git a/drivers/iio/accel/mma8452.c b/drivers/iio/accel/mma8452.c
index ba27f86..4e6e702 100644
--- a/drivers/iio/accel/mma8452.c
+++ b/drivers/iio/accel/mma8452.c
@@ -1580,7 +1580,7 @@ static int mma8452_probe(struct i2c_client *client,
 	case FXLS8471_DEVICE_ID:
 		if (ret == data->chip_info->chip_id)
 			break;
-		/* fall through */
+		fallthrough;
 	default:
 		ret = -ENODEV;
 		goto disable_regulators;