mac80211: make bridge_packets a virtual interface option

The bridge_packets configuration really should be per virtual
interface (theoretically per AP/VLAN, but this is much easier);
there currently is no way to set it yet though. Also invert
the option to "NO_BRIDGE_PACKETS" so the default is to bridge.

While at it, also document the flags properly.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>