iio: temperature: ltc2983: 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: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20220621202719.13644-26-jic23@kernel.org