net: dsa: fix dsa_is_normal_port helper

In order to know if a port is of type user, dsa_is_normal_port checks
that the given port is not of type DSA nor CPU. This is not enough
because a port can be unused.

Without the previous fix, this caused the unused mv88e6xxx ports to be
configured in normal mode.

The ds->enabled_port_mask reports the user ports, so check this instead.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>