rpmsg: glink: spi: Hold channel reference during send

Hold a refcount while clients are sending so the channel is not freed
during the send. This is important for when glink spi is waiting for
an intent but the wdsp processor crashes.

Change-Id: I0cb8dac6f12694441b24e804d86bbfeb3ec9eb6a
Signed-off-by: Chris Lew <clew@codeaurora.org>