bridge: Turn flag change macro into a function.

Turn the flag change macro into a function to allow
easier updates and to reduce space.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>