bridge: prevent bridging wrong device

The bridge code assumes ethernet addressing, so be more strict in
the what is allowed. This showed up when GRE had a bug and was not
using correct address format.

Add some more comments for increased clarity.

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