soc: qcom: spcom: release ion buffer on rpmsg channel remove

This change decreases reference count on shared memory buffers locked
by hlos application, because rpmsg channel device removed when secure
processor communication endpoint disconnected.  The reason for such is
ssr event.

Change-Id: Ifd3167101a7c20160d0e8bae98f79ab424db2390
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>