net: dsa: rtl8366rb: Support all 4096 VLANs

There is an off-by-one error in rtl8366rb_is_vlan_valid()
making VLANs 0..4094 valid while it should be 1..4095.
Fix it.

Fixes: d8652956 ("net: dsa: realtek-smi: Add Realtek SMI driver")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>