bridge: new mode flag to indicate mode 'undefined'

This patch adds mode BRIDGE_MODE_UNDEF for cases where mode is not needed.

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>