staging:iio:adc:ad7280a: Cleanup includes

Drop used includes, add a few missing ones and reorder.
The include-what-you-use tool output was considered in making this
change.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://lore.kernel.org/r/20220206190328.333093-12-jic23@kernel.org