net: dsa: mt7530: correct ds->num_ports

Setting ds->num_ports to DSA_MAX_PORTS made DSA core allocate unnecessary
dsa_port's and call mt7530_port_disable for non-existent ports.

Set it to MT7530_NUM_PORTS to fix that, and dsa_is_user_port check in
port_enable/disable is no longer required.

Cc: stable@vger.kernel.org
Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>