net: macb: write mac address when initialization

When boot up without mac address setting, it will give the warning
message like: "Warning: failed to set MAC address", however when
execute network related command, it still execute them without any
warning information.

With this patch, it will exit directly with following information:
"gmac0: mac address is not valid"

It also solve the problem after bootup then set mac address and the
mac address won't set to net device issue.

Signed-off-by: Bo Shen <voice.shen@atmel.com>