iio: ad5686: Remove redundant variable

‘regdone’ is not used in the function. Remove it.

Signed-off-by: Sachin Kamat <sachin.kamat@samsung.com>
Cc: Michael Hennerich <hennerich@blackfin.uclinux.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>