msm: mhi: Changes to avoid mhi_sm_wq schedule out

Queue mhi_sm_wq as system_highpri_wq to avoid WQ getting
scheduled out.

Change-Id: I92f6442a6425dc71b64550078cfc44d85f744f41
Signed-off-by: Subramanian Ananthanarayanan <skananth@codeaurora.org>
Signed-off-by: Gauri Joshi <gaurjosh@codeaurora.org>