net: ftgmac100: move phy connect out from ftgmac100_setup_mdio

Split MDIO registration and PHY connect into ftgmac100_setup_mdio and
ftgmac100_mii_probe.

Signed-off-by: Ivan Mikhaylov <i.mikhaylov@yadro.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>