The CPU port does not have a PHY connected to it. So calling
phy_support_asym_pause() results in an Opps. As with other DSA
drivers, add a guard that the port is a user port.

Reported-by: Michal Vokáč <michal.vokac@ysoft.com>
Fixes: 0394a63a

 ("net: dsa: enable and disable all ports")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Michal Vokáč <michal.vokac@ysoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>