staging: iio: resolver: replace iio_device_register by devm_iio_device_register

Use devm_iio_device_register instead of iio_device_register when the remove
function is only used to call iio_device_unregister in order to ease the error path.
Since resource managed functions implicitly call unregister at driver detach also remove
iio_device_unregister

Signed-off-by: Ioana Ciornei <ciorneiioana@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>