soc: qcom: glink_ssr: Validate subsystem info during channel close

In the glink_ssr client's close function, the subsystem information
structure is obtained using the get_info_for_edge() function. This
function returns NULL if no subsystem information structure is found for
the requested edge. The close function does not check whether NULL was
returned, which could lead to a perceived NULL pointer dereference.
However, this is not possible since the edge info is configured from the
device tree before the channel is opened and should always be valid.

Add check to ensure that the design assumption that get_info_for_edge()
is valid is still true.

Change-Id: If88cbc1419669fa240667ae92ce2e78e49ad970c
Signed-off-by: Steven Cahail <scahail@codeaurora.org>