net: dsa: loop: Set correct number of ports

We only support DSA_LOOP_NUM_PORTS in the switch, do not tell the DSA
core to allocate up to DSA_MAX_PORTS which is nearly the double (6 vs.
11).

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>