net: dsa: qca8k: Initialize the switch with correct number of ports

Since commit 0394a63a ("net: dsa: enable and disable all ports")
the dsa core disables all unused ports of a switch. In this case
disabling ports with numbers higher than QCA8K_NUM_PORTS causes that
some switch registers are overwritten with incorrect content.

To fix this, initialize the dsa_switch->num_ports with correct number
of ports.

Fixes: 7e99e347 ("net: dsa: remove dsa_switch_alloc helper")
Signed-off-by: Michal Vokáč <michal.vokac@ysoft.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>