Altera TSE: Move PHY get addr and MDIO create

Move PHY get addr and MDIO create to a new function to improve readability
and make it easier to avoid its usage. This will be useful for example in
the case where there is no PHY chip.

Signed-off-by: Walter Lozano <walter@vanguardiasur.com.ar>
Signed-off-by: David S. Miller <davem@davemloft.net>