rpmsg: glink: The mbox client knows_txdone

As the GLINK driver is ticking the txdone of the mailbox channel (to
implement the doorbell) it needs to set knows_txdone.

Change-Id: Ib6470f024ab4b5fb6cca57c09cb4921f85e2f2f6
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Patch-mainline: linux-arm-msm @ 15/11/2017 22:08
Signed-off-by: Chris Lew <clew@codeaurora.org>