net: dsa: Define max num of bridges in tag8021q implementation

Max number of bridges in tag8021q implementation is strictly limited
by VBID size: 3 bits. But zero is reserved and only 7 values can be used.

This patch adds define which describe maximum possible value.

Suggested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://patch.msgid.link/20240713211620.1125910-10-paweldembicki@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>