staging:iio:ad7298: Do not return error code in interrupt handler

The interrupt handler should only ever return one of the three irqreturn_t
constants and not an error code. Also make sure to always call
iio_trigger_notify_done before leaving the trigger handler.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>