net: dsa: b53: Fix default VLAN ID

We were not consistent in how the default VID of a given port was
defined, b53_br_leave() would make sure the VLAN ID would be either 0/1
depending on the switch generation, but b53_configure_vlan(), which is
the default configuration would unconditionally set it to 1. The correct
value is 1 for 5325/5365 series and 0 otherwise. To avoid repeating that
mistake ever again, introduce a helper function: b53_default_pvid() to
factor that out.

Fixes: 967dd82f ("net: dsa: b53: Add support for Broadcom RoboSwitch")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>