net: ravb: Switch to SYSTEM_SLEEP_PM_OPS()/RUNTIME_PM_OPS() and pm_ptr()

SET_SYSTEM_SLEEP_PM_OPS() and SET_RUNTIME_PM_OPS() are deprecated now
and require __maybe_unused protection against unused function warnings.
The usage of pm_ptr() and SYSTEM_SLEEP_PM_OPS()/RUNTIME_PM_OPS() allows
the compiler to see the functions, thus suppressing the warning. Thus
drop the __maybe_unused markings.

Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>