PM / QoS: Properly use the WARN() macro in dev_pm_qos_add_request()

Make dev_pm_qos_add_request() use WARN() in a better way and do not hardcode
the function's name into the message (use __func__ instead).

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>