slim-msm-ngd: Prevent race condition between resume and SSR

Prevents the possible race condition between slimbus resume
and slimbus SSR function execution. When SSR callback function
takes mutex lock ahead of resume function, it results in qmi
NULL pointer dereference in resume function as SSR function
execution makes it NULL.

Change-Id: I531d62077329624cf09044235859a2db328892a0
Signed-off-by: Prudhvi Yarlagadda <pyarlaga@codeaurora.org>