net: fec: ensure that a disconnected phy isn't configured

When we disconnect from a phy, we should forget our pointer to it so we
don't accidentally try to configure it.  We handle a NULL phy pointer
correctly in most places, except fec_enet_set_pauseparam().  Fix this
too.

Acked-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>