dpaa_eth: perform DMA unmapping before read

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>