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.

Acked-By: Chris Lew <clew@codeaurora.org>
Reported-by: Arun Kumar Neelakantam <aneela@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>