net: bridge: vlan tunnel: constify bridge and port arguments

The vlan tunnel code changes vlan options, it shouldn't touch port or
bridge options so we can constify the port argument. This would later help
us to re-use these functions from the vlan options code.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>