Bridge family: Automatically adjust MTU to include MACsec header

MACsec adds a header to packets passing through interfaces where MACsec is enabled. If a packet is near the protocol MTU limit, and the MTU is not adjusted to account for the MACsec header, the packet can exceed the interface MTU when the MACsec header is added. In that case, the device drops the packet. Before enabling MACsec, you must ensure your protocol MTU is large enough to accommodate the additional 32 bytes of MACsec overhead.