net: fec: add netif status check before set mac address

There exist one issue by below case that case system hang:
ifconfig eth0 down
ifconfig eth0 hw ether 00:10:19:19:81:19

After eth0 down, all fec clocks are gated off. In the .fec_set_mac_address()
function, it will set new MAC address to registers, which causes system hang.

So it needs to add netif status check to avoid registers access when clocks are
gated off. Until eth0 up the new MAC address are wrote into related registers.

V2:
As Lucas Stach's suggestion, add a comment in the code to explain why it needed.

CC: Lucas Stach <l.stach@pengutronix.de>
CC: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>