iio: imu: adis: assign value only if return code zero in read funcs

The inline read functions in the ADIS library don't check the return value
of the `adis_read_reg()` function and assign the value of `tmp` regardless.

Fix this by checking if return value is zero and only then assigning the
value of `tmp`.

No known case of the callers of this function incorrectly using the
value, but best to stop any potential risk here.
Not suitable for stable due to no known actual bugs caused by this
issue.

Fixes: 57a1228a ("iio:imu:adis: Add support for 32bit registers")
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>