staging: iio: ad7192: rename regulator 'reg' to 'avdd'

Rename regulator 'reg' to 'avdd' so as to be clear what regulator it
stands for specifically. Also, update the goto label accordingly.

Signed-off-by: Eva Rachel Retuya <eraretuya@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>