iio:adc:at91-adc: Switch from CONFIG_PM_SLEEP guards to pm_sleep_ptr() etc

Letting the compiler remove these functions when the kernel is built
without CONFIG_PM_SLEEP support is simpler and less error prone than the
use of ifdef based config guards.

Removing instances of this approach from IIO also stops them being
copied into new drivers.

Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20220130193147.279148-11-jic23@kernel.org