net/faraday: Mask PHY interrupt with NCSI mode

Bogus PHY interrupts are observed. This masks the PHY interrupt
when the interface works in NCSI mode as there is no attached
PHY under the circumstance.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Acked-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: David S. Miller <davem@davemloft.net>