iio: light: tsl2591: Use DEFINE_RUNTIME_DEV_PM_OPS() and pm_ptr()

Using these new macros allows removal of unused pm_ops structure and
functions if !CONFIG_PM without the need to mark the functions
__maybe_unused.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Joe Sandom <joe.g.sandom@gmail.com>
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20220621202719.13644-32-jic23@kernel.org