net: dsa: qca8k: Use up to 7 ports for all operations

The QCA8K family supports up to 7 ports. So use the existing
QCA8K_NUM_PORTS define to allocate the switch structure and limit all
operations with the switch ports.

This was not an issue until commit 0394a63a ("net: dsa: enable and
disable all ports") disabled all unused ports. Since the unused ports 7-11
are outside of the correct register range on this switch some registers
were rewritten with invalid content.

Fixes: 6b93fb46 ("net-next: dsa: add new driver for qca8xxx family")
Fixes: a0c02161 ("net: dsa: variable number of ports")
Fixes: 0394a63a

 ("net: dsa: enable and disable all ports")
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>