PM / QoS: Add return code to pm_qos_get_value function.

pm_qos_get_value don't return a return code in all cases. It's sure that
anything interesting happend after BUG() but this prevent any compilation
warning.

[rjw: Chaneged the new return value to PM_QOS_DEFAULT_VALUE.]

Signed-off-by: Luis Gonzalez Fernandez <luisgf@gmail.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>