iio: light: bh1750: simplify setting PM ops

Relying on CONFIG_PM_SLEEP to set PM ops is not necessary
since core will handle everything internally. One have to only make sure
that functions that can go unused are marked with __maybe_unused.

Signed-off-by: Tomasz Duszynski <tduszyns@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>