iio: gyro: adis16136: check ret val for non-zero vs less-than-zero

The ADIS library functions return zero on success, and negative values for
error. Positive values aren't returned, but we only care about the success
value (which is zero).

This change is mostly needed so that the compiler won't make any inferences
about some about values being potentially un-initialized. This only
triggers after making some functions inline, because the compiler can
better follow return paths.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>