dpaa_eth: perform DMA unmapping before read

[ Upstream commit c70fd318 ]

DMA unmapping is required before accessing the HW provided timestamping
information.

Fixes: 4664856e ("dpaa_eth: add support for hardware timestamping")
Signed-off-by: Madalin Bucur <madalin.bucur@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>