bridge: T6125: support 802.1ad (ethertype 0x88a8) VLAN filtering

Linux bridge uses EtherType 0x8100 by default. In some scenarios, an EtherType
value of 0x88A8 is required.

Reusing CLI command from VIF-S (QinQ) interfaces:

set interfaces bridge br0 protocol 802.1ad

(cherry picked from commit 9c9b1febff6863ccd3632a04d9e307909b3efe7a)