ASoC: psc-ac97: constify dev_pm_ops structure

This dev_pm_ops structure is only stored in the pm field of a
device_driver structure. This field is declared const, so
dev_pm_ops structures that have this property can be declared
as const also.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Mark Brown <broonie@kernel.org>