soc: qcom: glink: Fix memory leak in G-Link SSR channel close

When a G-Link SSR channel is created, memory is allocated for private
callback data for the channel. This memory is not freed when the channel
is closed.

Free the memory when the channel is closed.

Change-Id: I13c888367605db78895cc5a3cf63e74fb1326ef2
Signed-off-by: Steven Cahail <scahail@codeaurora.org>