rpmsg: qcom_smd: Ensure ordering of channel info updates

Add wmb() call support after data is written to fifo and
before head index is updated to ensure that data is
written to fifo before head index updation.

Change-Id: Ia2937dc7de28ae31d0d759bf15b869b6f8fb93fe
Signed-off-by: Vivek Golani <vgolani@codeaurora.org>