msm: ipa: Mutex protect state variable

This fix mutex protects a IPA MHI manager state variable
for synchronization.

Change-Id: I107226467668466f0c020ada65888a9641dac4c7
Acked-by: Jyothi Jayanthi <jyothij@qti.qualcomm.com>
Signed-off-by: Michael Adisumarta <madisuma@codeaurora.org>