iio:adc:ad7923 move to info_mask_(shared_by_type/separate)

The original info_mask is going away in favour of the broken out versions.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
cc: Patrick Vasseur <patrick.vasseur@c-s.fr>
cc: Christophe Leroy <christophe.leroy@c-s.fr>