msm: ipa: fix race condition between USB connect and modem SSR

This fix solves the race conditions between USB connect and
modem SSR by using mutex protection.

Change-Id: If97264a7c875f3e0c66b34b1b02e48b21c99e319
Acked-by: Jyothi J <jyothij@qti.qualcomm.com>
Signed-off-by: Michael Adisumarta <madisuma@codeaurora.org>