net: dsa: mv88e6xxx: Remove OF check for IRQ domain

An IRQ domain will work without an OF node. It is not possible to
reference interrupts via a phandle, but C code can still use
irq_find_mapping() to get an interrupt from the domain.

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