phy: fix the use of PHY_IGNORE_INTERRUPT

When a PHY device is registered with the special IRQ value
PHY_IGNORE_INTERRUPT (-2) it will not properly be handled by the PHY
library:

- it continues to poll its register, while we do not want this
  because such PHY link events or register changes are serviced by an
  Ethernet MAC
- it will still try to configure PHY interrupts at the PHY level, such
  interrupts do not exist at the PHY but at the MAC level
- the state machine only handles PHY_POLL, but should also handle
  PHY_IGNORE_INTERRUPT similarly

This patch updates the PHY state machine and initialization paths to
account for the specific PHY_IGNORE_INTERRUPT. Based on an earlier patch
by Thomas Petazzoni, and reworked to add the missing bits. Add a helper
phy_interrupt_is_valid() which specifically tests for a PHY interrupt
not to be PHY_POLL or PHY_IGNORE_INTERRUPT and use it throughout the
code.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>