Instead of doing the dance with macro that either resolves to a pointer or
NULL, let's switch to using SIMPLE_DEV_PM_OPS().

Also let's mark suspend and resume methods as __maybe_unused instead of
guarding them with an #ifdef and rely on linker to drop unused code. Doing
so should allow better compile coverage.
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>