iio: adc: palmas_gpadc: fix NULL dereference on rmmod

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>