net: dsa: Fix inverted test for multiple CPU interface

Remove the wrong !, otherwise we get false positives about having
multiple CPU interfaces.

Fixes: b22de490 ("net: dsa: store CPU switch structure in the tree")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>