staging:iio:ad7291 faciliate ad7291_read_event_config()

Save some lines in returning a voltage event.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>