iio:adc:ad799x: Allow writing of event config

Allows explicity enabling of events

Previously, events were always reported as enabled, but actually only
implicitly enabled when updating the buffer scan mode

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