staging: iio: ad9832: clean-up regulator 'reg'

Rename regulator 'reg' to 'avdd' so as to be clear what regulator it
stands for specifically. Additionally, get rid of local variable 'reg'
and use direct assignment instead. Update also the goto label pertaining
to the avdd regulator during disable.

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