iio:magnetometer:ak8975: fix missing regulator_disable

Ensure optional regulator is properly disabled when present.

Fixes: 63d5d525 ("iio:magnetometer:ak8975: power regulator support")
Signed-off-by: Gregor Boirie <gregor.boirie@parrot.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>