Documentation: networking: dsa: remove static port count from limitations

After Vivien's series from 2019 containing commits 27d4d19d ("net:
dsa: remove limitation of switch index value") and ab8ccae1 ("net:
dsa: add ports list in the switch fabric"), this is basically no longer
true.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>