Micrel phy KSZ9021 Gb speed cannot work well in i.MX6 sabrelite
board. Advertise PHY is not 1000Base-T capable.

If nfs boot kernel, phy will work at 100Mbps, or else phy will
work at Gbps mode. And if hot-plugin cable, phy will work at Gbps
mode. Enet can work well in all of them.

But in Gbps mode, uDMA sometime cannot write  frame "L" bit in
exuberate transfer, so driver will print some warning message:
"FEC ENET: rcv is not +last"

Signed-off-by: Fugang Duan <B38611@freescale.com>