net: thunderx: Cleanup PHY probing code.

Remove the call to force the octeon-mdio driver to be loaded.  Allow
the standard driver loading mechanisms to load the PHY drivers, and
use -EPROBE_DEFER to cause the BGX driver to be probed only after the
PHY drivers are available.

Reorder the setting of MAC addresses and PHY probing to allow BGX
LMACs with no attached PHY to still be assigned a MAC address.

Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>