staging:iio:adis16260: Add proper range checks to write_frequency()

A negative sampling frequency is obviously invalid, so use kstrtouint() instead
of strict_strtoul. Also when setting a sampling frequency smaller than the
minimum supported frequency set the frequency to the minimum supported frequency
instead of just cutting off the upper bits of the raw register value.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>