net: dsa: Fix dsa_is_user_port() test inversion

During the conversion to dsa_is_user_port(), a condition ended up being
reversed, which would prevent the creation of any user port when using
the legacy binding and/or platform data, fix that.

Fixes: 4a5b85ff

 ("net: dsa: use dsa_is_user_port everywhere")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>