iio: frequency: admv1014: return -EINVAL directly

Remove extra step where the error code is assigned to the `ret`
variable.

Return instead error code directly.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Link: https://lore.kernel.org/r/20220819104117.4600-1-antoniu.miclaus@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>