net: fec: correct phy_name buffer length when init phy_name

Fix the bug that we got wrong phy_name on imx6q sabrelite board.
snprintf used wrong length of phy_name.
phy_name length is MII_BUS_ID_SIZE + 3 rather not MII_BUS_ID_SIZE.
I change it to sizeof(phy_name).

Signed-off-by: Richard Zhao <richard.zhao@linaro.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>