power: qos: Fix accessing pm_qos_request struct

Constraints added using dev_pm_qos_add_request are not member
of pm_qos_request struct.

Only those constraints that are added using pm_qos_add_request
are member of pm_qos_request struct and can be accessed as CPU DMA
latency constraints while updating the cpumask for which latency
has been updated.

Add a check if constraint is of CPU DMA latency class before
accessing.

Change-Id: I809b8fc46379311902cd3eea4e55f753e9002c69
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>