Staging: iio: Simplify return logic

This patch removes the conditional return of the ade77*_probe functions
based on the return values of iio_device_register as the latter
returns 0 or ret, the same as the checked values.

Warning found by coccinelle.

Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Reviewed-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>