nl80211: Add a missing break in parse_station_flags

I was looking at usually suppressed gcc warnings,
[-Wimplicit-fallthrough=] in this case:

The code definitely looks like a break is missing here.
However I am not able to test the NL80211_IFTYPE_MESH_POINT,
nor do I actually know what might be :)
So please use this patch with caution and only if you are
able to do some testing.

Signed-off-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
[johannes: looks obvious enough to apply as is, interesting
 though that it never seems to have been a problem]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>