generic: ar8216: fix invalid bounds check imported from ChromeOS (FS#347)

The priv->vlan_id member is of size AR8X16_MAX_VLANS, not AR8X16_MAX_PORTS,
so check for the proper maximum value in order to avoid capping valid VLAN IDs
to 7 (AR8X16_MAX_PORTS - 1).

Signed-off-by: Jo-Philipp Wich <jo@mein.io>