net: fec: Avoid MX28 bus sync issue

The MX28 multi-layer AHB bus can be too slow and trigger the
FEC DMA too early, before all the data hit the DRAM. This patch
ensures the data are written in the RAM before the DMA starts.
Please see the comment in the patch for full details.

This patch was produced with an amazing help from Albert Aribaud,
who pointed out it can possibly be such a bus synchronisation
issue.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>
Tested-by: Alexandre Pereira da Silva <aletes.xgr@gmail.com>