hwrng: omap - remove #ifdefery around PM methods

Instead of using #ifdefs let's mark suspend and resume methods as
__maybe_unused which will suppress compiler warnings about them being
unused and provide better compile coverage.

Because SIMPLE_DEV_PM_OPS() produces an empty omap_rng_pm structure in
case of !CONFIG_PM_SLEEP neither omap_rng_suspend nor omap_rng_resume
will end up being referenced and the change will not result in
increasing image size.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>