msm: ipa: Add mutex protection when updating notify req cache

IOCTL interface to send QMI NOTIFY REQ messages can be called
from multiple contexts which can corrupt the notify req cache.
Make a change to add mutext protection to prevent this
corruption.

Change-Id: Ib6d879e29971ae40398a9dc8759fb4dbbf166429
Acked-by: Chaitanya Pratapa <cpratapa@qti.qualcomm.com>
Signed-off-by: Utkarsh Saxena <usaxena@codeaurora.org>