Input: wistron - fix test for CONFIG_PM

This should fix the following compile warning:

drivers/input/misc/wistron_btns.c:1331:5: warning: "CONFIG_PM" is not defined

Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>