staging: iio: adc: convert ADI drivers to use kfifo.

sw_ring is depreciated and therefore won't move out of staging.
Prerequisite for lifting affected drivers is to convert them to kfifo.
Update copyright.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>