msm:ipa: mutex protect mhi dev for voting/unvoting pcie clocks

This change mutex protects mhi dev for pcie clock voting/unvoting
in concurrency scenarios such as USB disconnect and SSR.
Not protecting may cause invalid mhi dev access.

Change-Id: Ib3745068a94a8e15fc61c7dfd4245c97e777c9ae
Signed-off-by: Michael Adisumarta <madisuma@codeaurora.org>