radiotap: fix vendor namespace parsing

There's a bug with radiotap vendor namespace
parsing if you don't register for the given
namespace extensions. Fix this by passing
only the unknown vendor namespaces and the
registered data to frontends, but not both.

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