msm: gsi: fix interrupt processing

Fix GSI interrupt processing to make sure interrupt are
not missed. In order to achieve that interrupt should first be
cleared before processed.

Change-Id: I42978f2230e95456e4b4e932365e5b2c83445f56
CRs-Fixed: 1090894
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>