iio: adc: palmas_gpadc: fix NULL dereference on rmmod

[ Upstream commit 49f76c499d38bf67803438eee88c8300d0f6ce09 ]

Calling dev_to_iio_dev() on a platform device pointer is undefined and
will make adc NULL.

Signed-off-by: Patrik Dahlström <risca@dalakolonin.se>
Link: https://lore.kernel.org/r/20230313205029.1881745-1-risca@dalakolonin.se


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>