staging: iio: cdc: ad7746: clean up probe return

Slight simplication of the probe return on device register.

Signed-off-by: Lucas Stankus <lucas.p.stankus@gmail.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/45443b8306893576824effaff57d40231de8e813.1621786036.git.lucas.p.stankus@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>