adsprpc: set QoS only to silver cluster

Glink IRQ mostly is taken by the silver cluster. RPC driver vote
for sliver cluster prevents collapsing gold cluster. This saves
significant power.

Change-Id: Ic24bddceb7ae37d1182d2fca683c622b4ab71a55
Acked-by: Tadakamalla Krishnaiah <ktadakam@qti.qualcomm.com>
Signed-off-by: Mohammed Nayeem Ur Rahman <mohara@codeaurora.org>