mac80211: fix mesh advertisement

When a mac80211-based driver advertises mesh mode
support, this will be advertised to userspace.
However, if mac80211 was compiled without mesh
support, then that won't actually be true. Fix
this by removing the bit for mesh if mesh isn't
compiled in.

Since this synchronizes what we advertise to
cfg80211 and actually support, it means we can
now rely on cfg80211's interface type checks
and need not check again in mac80211.

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