Monitor ring Rx buffer is not really modified between dma map
and unmap. So remove the unnecssary data sync before dma unmap.
This does not fix any visible issue, found in code review.

Compile tested only.
Signed-off-by: Vasanthakumar Thiagarajan <vthiagar@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1602744454-22969-1-git-send-email-vthiagar@codeaurora.org