Input: st-keyscan - fix 'defined but not used' compiler warnings

Add #ifdef CONFIG_PM_SLEEP around keyscan_supend() and keyscan_resume() to
fix the following compiler warnings occuring if CONFIG_PM_SLEEP is unset:

  + /scratch/kisskb/src/drivers/input/keyboard/st-keyscan.c: warning: 'keyscan_resume' defined but not used [-Wunused-function]:  => 235:12
  + /scratch/kisskb/src/drivers/input/keyboard/st-keyscan.c: warning: 'keyscan_suspend' defined but not used [-Wunused-function]:  => 218:12

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lkml.org/lkml/2014/7/8/109


Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>