net: dsa: rtl8366: Fix up VLAN filtering

We get this regression when using RTL8366RB as part of a bridge
with OpenWrt:

WARNING: CPU: 0 PID: 1347 at net/switchdev/switchdev.c:291
	 switchdev_port_attr_set_now+0x80/0xa4
lan0: Commit of attribute (id=7) failed.
(...)
realtek-smi switch lan0: failed to initialize vlan filtering on this port

This is because it is trying to disable VLAN filtering
on VLAN0, as we have forgot to add 1 to the port number
to get the right VLAN in rtl8366_vlan_filtering(): when
we initialize the VLAN we associate VLAN1 with port 0,
VLAN2 with port 1 etc, so we need to add 1 to the port
offset.

Fixes: d8652956

 ("net: dsa: realtek-smi: Add Realtek SMI driver")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>