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>