This symbol is not used outside of cm32181.c, so marks it static.

Fixes the following sparse warnings:
>> drivers/iio/light/cm32181.c:508:1: sparse: sparse: symbol 'cm32181_pm_ops'
was not declared. Should it be static?
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: sunliming <sunliming@kylinos.cn>
Link: https://lore.kernel.org/r/20220815012930.150078-1-sunliming@kylinos.cnSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>