iio: sca3000: Fix missing return in switch

The IIO_CHAN_INFO_LOW_PASS_FILTER_3DB_FREQUENCY case is missing a
return and will fall through to the default case and errorenously
return -EINVAL.

Fix this by adding in missing *return ret*.

Fixes: 626f971b ("staging:iio:accel:sca3000 Add write support to the low pass filter control")
Reported-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>