iio: dac: max5821: Switch to DEFINE_SIMPLE_DEV_PM_OPS() and pm_sleep_ptr()

Using these newer macros allows the compiler to remove the unused
structure and functions when !CONFIG_PM_SLEEP + removes the need to
mark pm functions __maybe_unused.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Philippe Reynes <tremyfr@yahoo.fr>
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20220621202719.13644-15-jic23@kernel.org