net: dsa: b53: fix VLAN_ID_IDX write size for BCM5325/65

net: dsa: b53: fix VLAN_ID_IDX write size for BCM5325/65

Since BCM5325 and BCM5365 only support up to 256 VLANs, the VLAN_ID_IDX
register is only 8 bit wide, not 16 bit, so use an appropriate accessor.

Fixes: c45655386e53 ("net: dsa: b53: add support for FDB operations on 5325/5365")
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Tested-by: Álvaro Fernández Rojas <noltari@gmail.com>
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Link: https://patch.msgid.link/20251128080625.27181-2-jonas.gorski@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>