net: fec: clear all interrupt events to support i.MX6SX

For i.MX6SX FEC controller, there have interrupt mask and event
field extension. To support all SOCs FEC, we clear all interrupt
events during MAVC initial process.

Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>