Input: mpr121 - annotate PM methods as __maybe_unused

Instead of using #ifdef, let's mark suspend and resume methods as
__maybe_unused to provide better compile coverage.

Suggested-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>