msm: mhi_dev: check for error during ring processing

The change is to check for error code from cmd processing in case
of db pending cases, to avoid rd_offset increment.

Change-Id: If213297fe02ec1087bfab572f5af97b1e7bb5791
Signed-off-by: Subramanian Ananthanarayanan <skananth@codeaurora.org>
Signed-off-by: Gauri Joshi <gaurjosh@codeaurora.org>