net: dsa: mt7530: use priv->ds->num_ports instead of MT7530_NUM_PORTS

Use priv->ds->num_ports on all for loops which configure the switch
registers. In the future, the value of MT7530_NUM_PORTS will depend on
priv->id. Therefore, this change prepares the subdriver for a simpler
implementation.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: David S. Miller <davem@davemloft.net>