net: force bridge module(s) to be GPL

The only valid usage for the bridge frame hooks are by a
GPL components (such as the bridge module).
The kernel should not leave a crack in the door for proprietary
networking stacks to slip in.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>