Input: stmfts - mark all PM functions as __maybe_unused

We get a harmless warning when CONFIG_RUNTIME_PM is disabled:

drivers/input/touchscreen/stmfts.c:760:12: error: 'stmfts_runtime_resume' defined but not used [-Werror=unused-function]
 static int stmfts_runtime_resume(struct device *dev)
drivers/input/touchscreen/stmfts.c:748:12: error: 'stmfts_runtime_suspend' defined but not used [-Werror=unused-function]
 static int stmfts_runtime_suspend(struct device *dev)

The regular PM functions are already marked as __maybe_unused, so let's
do the same for the runtime-PM as well.

Fixes: 78bcac7b

 ("Input: add support for the STMicroelectronics FingerTip touchscreen")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>