net: phy: start interrupts in phy_start

Interrupts don't have to be enabled before calling phy_start().
Therefore let's enable them in phy_start(). In a subsequent step
we'll remove enabling interrupts from phy_connect_direct().

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>