net: mvneta: disable MVNETA_CAUSE_PSC_SYNC_CHANGE interrupt

The PSC sync change interrupt can fire multiple times while the link is
down, which is caused by noise on the serdes lines. As this isn't
information we make use of, it's pointless having the interrupt enabled.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>