Input: adp5589-keys - mark suspend and resume methods as __maybe_unused

This improves compile coverage of the code; unused code will be dropped by
the linker.

Acked-by: Alexandru Ardelean <Alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20201119072418.GA114677@dtor-ws


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>