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

IOCTL interface to send QMI NOTIFY REQ messages can be called
from multiple contexts which can result into buffer overflow of
msg cache. Make a change to add mutext protection to prevent
buffer overflow.

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