iio:gyro: adis16136: divide by zero in write_frequency()

It's slightly cleaner to use kstrtouint() because we pass unsigned ints
to adis16136_set_freq().  On 64 bit systems, if the user passed LONG_MIN
then it we would get past the test against zero but crash in
adis16136_set_freq() because we truncate the high bits away.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-By: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>