PM / QoS: Use NULL pointer instead of plain integer in pm_qos.h

Fix the following sparse warning:
include/linux/pm_qos.h:69:28: warning: Using plain integer as NULL pointer

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>