bridge: Consolidate error paths in br_add_bridge().

This actually had to be merged with the patch #1, but I decided not to
mix two changes in one patch.

There are already two calls to free_netdev() in there, so merge them
into one.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>