iio: light: cm32181: Mark the dev_pm_ops static.

Only accessed from the local file.
Warning:
  drivers/iio/light/cm32181.c:508:1: warning: symbol 'cm32181_pm_ops' was not declared. Should it be static?

Fixes: 68c1b3dd

 ("iio: light: cm32181: Add PM support")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Kai-Heng Feng <kai.heng.feng@canonical.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220807145457.646062-3-jic23@kernel.org