msm: ipa: add a missing mutex unlock for IMP

Add a missing mutex unlock tp IPA MHI Proxy when
clock vote request arrives.

Change-Id: I010bebce0254bec8d3afc453237bef66bbeb212e
CRs-Fixed: 2253877
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>