net: phy: improve phy_driver callback handle_interrupt

did_interrupt() clears the interrupt, therefore handle_interrupt() can
not check which event triggered the interrupt. To overcome this
constraint and allow more flexibility for customer interrupt handlers,
let's decouple handle_interrupt() from parts of the phylib interrupt
handling. Custom interrupt handlers now have to implement the
did_interrupt() functionality in handle_interrupt() if needed.

Fortunately we have just one custom interrupt handler so far (in the
mscc PHY driver), convert it to the changed API.

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