msm: gsi: fix event ring handle in gsi_alloc_channel

Add a check to event ring handle.

Change-Id: If835e7e610745aeb82d4d0e363ef833d0f8d857a
CRs-Fixed: 1115476
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>