PM QoS: Fix inline documentation.

Fix the pm_qos_add_request() kerneldoc comment that doesn't reflect
the behavior of the function after the last PM QoS update.

Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
Acked-by: mark gross <markgross@thegnar.org>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>