net: phy: change phy_start_interrupts to phy_request_interrupt

Now that we enable the interrupts in phy_start() we don't have to do it
before. Therefore remove enabling interrupts from phy_start_interrupts()
and rename this function to reflect the changed functionality.

v2:
- improve warning to clearly state that we fall back to polling

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