net: dsa: qca8k: Add define for port VID

Rather than using a magic value of 1 when configuring the port VIDs add
a QCA8K_PORT_VID_DEF define and use that instead. Also fix up the
bitmask in the process; the top 4 bits are reserved so this wasn't a
problem, but only masking 12 bits is the correct approach.

Signed-off-by: Jonathan McDowell <noodles@earth.li>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>