net: phy: enable interrupts when PHY is attached already

This patch is a step towards allowing PHY drivers to handle more
interrupt sources than just link change. E.g. several PHY's have
built-in temperature monitoring and can raise an interrupt if a
temperature threshold is exceeded. We may be interested in such
interrupts also if the phylib state machine isn't started.
Therefore move enabling interrupts to phy_request_interrupt().

v2:
- patch added to series

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>