iio: frequency: admv1013: remove the always true condition

unsigned int variable is always greater than or equal to zero. Make the
if condition simple.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Fixes: da35a7b5 ("iio: frequency: admv1013: add support for ADMV1013")
Link: https://lore.kernel.org/r/YdS3gJYtECMaDDjA@debian-BULLSEYE-live-builder-AMD64


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