mac80211: remove ancient reference to master interface

The master interface no longer exists ... and hasn't for
a few years now, so remove this reference :-)
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/iface.c

 /*
- * Interface handling (except master interface)
+ * Interface handling
  *
  * Copyright 2002-2005, Instant802 Networks, Inc.
  * Copyright 2005-2006, Devicescape Software, Inc.