net: macb: Reorganize macb_mii bringup

The macb mii setup (mii_probe() and mii_init()) previously was
somewhat interspersed, likely a result of organic growth and hacking.

This change moves mii bus registration into mii_init and probing the
bus for devices into mii_probe.

Signed-off-by: Brad Mouring <brad.mouring@ni.com>
Suggested-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>