iio: bd79124: Use set_rv and set_multiple_rv

The new GPIO callbacks 'set_rv' and 'set_multiple_rv' allow returning a
success code to indicate failures when setting GPIO status. Use them to
allow callers to know when things go south.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://patch.msgid.link/Z_N_J52IZ2IaWawl@mva-rohm


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