iio: adc: xilinx: use more devres helpers and remove remove()

In order to simplify resource management and error paths in probe() and
entirely drop the remove() callback - use devres helpers wherever
possible. Define devm actions for cancelling the delayed work and
disabling the clock.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Tested-by: Anand Ashok Dumbre <anandash@xilinx.com>
Reviewed-by: Anand Ashok Dumbre <anandash@xilinx.com>
Link: https://lore.kernel.org/r/20201130142759.28216-4-brgl@bgdev.pl
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>