net: fec: add ERR007885 for i.MX6ul enet IP

The errata ERR007885 HW fix don't add to i.MX6ul ENET IP version,
so add sw workaroud for the chip.

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>