staging:iio:ad7606: Let common remove function take a struct device *

Currently the common remove function takes a struct iio_dev *. This
parameter is retrieved by the individual driver remove functions by calling
get_drvdata() on their device. To simplify the code let the common remove
function directly take a struct dev * and do the IIO device in retrieval
the common remove function.

This also aligns the interface with the common probe function.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>