rpmsg: glink: smem: Ensure ordering during tx

Ensure the ordering of the fifo write and the update of the write index,
so that the index is not updated before the data has landed in the fifo.

Change-Id: I90061b0f6d81105b47f45274d69f00d846d2a1ce
Acked-By: Chris Lew <clew@codeaurora.org>
Reported-by: Arun Kumar Neelakantam <aneela@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Patch-mainline: linux-arm-msm @ 14/12/2017 12:15
Signed-off-by: Chris Lew <clew@codeaurora.org>