soc: qcom: glink: Fix NULL pointer dereference in glink_ssr notification

In the notify_for_subsystem() function, a NULL pointer dereference can
occur if a subsystem never came up when the system booted. In this case,
the driver will correctly detect that the subsystem did not come up and
will not create a channel or callback data for it. Later, when resetting
state after performing the notifications and receiving the responses,
notify_for_subsystem() assumes that callback data is present for all
subsystems, which leads to the NULL pointer dereference.

Before resetting state held in the callback data, ensure that it is not
NULL.

CRs-Fixed: 900599
Change-Id: I00e2b30bfea78e4193054a235c82c287c300498a
Signed-off-by: Steven Cahail <scahail@codeaurora.org>