msm: mhi: Fixed race condition in rmnet

Fix race condition in MHI RmNet whereby the outbound
MHI ring could be empty while the network stack believes
it is full. This leads to a non recoverable data stall.

Add rwlock to ensure checks for full and empty conditions
are atomic.

CRs-Fixed: 733646
Change-Id: If54baa4be762a976d64213d0ec52e969f2497036
Signed-off-by: Andrei Danaila <adanaila@codeaurora.org>