soc: qcom: glink_ssr: Set NULL after free

In function close_ch_worker variable ss_info->cb_data is not set to
NULL after free, this results in crash due to  use after free in
notify_for_subsystem function.

This variable is set to NULL to avoid any crash due to use after free.

CRs-Fixed: 2000384
Change-Id: Ia0c7b52e4c8f5d0215fd0ebfbca5e823530f205c
Signed-off-by: Dhoat Harpal <hdhoat@codeaurora.org>