iio: imu: adis: assign read val in debugfs hook only if op successful

This was also caught by the `-Wmaybe-uninitialized` warning, which
(ironically as-is) it makes quite a lot of sense to do for this.

The code that actually calls this function will fail to copy on
the uninitialized value.   Hence, patch does not need to go into stable.

Fixes: 78026a6f ("iio:imu:adis: Add debugfs register access support")
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>