msm: rpc: hold spinlock while doing wait_event

To prevent a race condition between wake_up called by
msm_rpcrouter_xprt_notify and wait_event in rr_read
hold a spinlock irqsave to prevent the system from going into
sleep when there is pending SMD data to process.

CRs-Fixed: 393416
Signed-off-by: Angshuman Sarkar <angshuman@codeaurora.org>
(cherry picked from commit 902de08f6670ddba147609e8b9f895dadf2275d9)

Change-Id: Ib7d8a868e1e968fcc8eeca53e2c1c2eeec8dd965
Signed-off-by: Sridhar Gujje <sgujje@codeaurora.org>