Input: wacom - fix compiler warning if !CONFIG_PM

If CONFIG_PM is not set:

drivers/hid/wacom_sys.c:1436: warning: ‘wacom_reset_resume’ defined but
not used

Protect the unused functions by #ifdef CONFIG_PM to fix this.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>