iio:ad7923: Return error if we didn't get the expected result

Instead of leaving 'val' uninitialized return an error if the result's address
did not match that of the channel we were trying to read.

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