slim-msm-ngd: Prevent race condition for suspend and SSR

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

Change-Id: I87ed0ecaeda9627552b6061f6b135740da067774
Signed-off-by: Prudhvi Yarlagadda <pyarlaga@codeaurora.org>