staging: iio: ad7606: implement IIO_CHAN_INFO_OVERSAMPLING_RATIO

This driver predates the availability of IIO_CHAN_INFO_OVERSAMPLING_RATIO
attribute wherein usage has some advantages like it can be accessed by
in-kernel consumers as well as reduces the code size.

Therefore, use IIO_CHAN_INFO_OVERSAMPLING_RATIO to implement the
oversampling_ratio attribute instead of using IIO_DEVICE_ATTR() macro.

Move code from the functions associated with IIO_DEVICE_ATTR() into
the read_raw hook as well as add the write_raw hook with both masks set
to IIO_CHAN_INFO_OVERSAMPLING_RATIO.

Signed-off-by: Eva Rachel Retuya <eraretuya@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>