Input: bu21013_ts - annotate supend/resume methods as __maybe_unused

Instead if #ifdef-ing out suspend and resume methods, let's mark
them as __maybe_unused to get better compile time coverage.

Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>