net: phy: icplus: add config_intr callback

Move IRQ configuration for IP101A/G from config_init to config_intr
callback. Reasons:

1. This allows phylib to disable interrupts if needed.
2. Icplus was the only driver supporting interrupts w/o defining a
   config_intr callback. Now we can add a phylib plausibility check
   disabling interrupt mode if one of the two irq-related callbacks
   isn't defined.

I don't own hardware with this PHY, and the change is based on the
datasheet for IP101A LF (which is supposed to be register-compatible
with IP101A/G). Change is compile-tested only.

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