cfg80211: make WEXT compatibility unselectable

This option has been marked for deprecation and removal for
a little more than two years, but it's not been very clearly
signalled since it was always possible to just select it.

Make it unselectable now to signal anyone who's still using
it after all this time more clearly. They can still get it
back, but only by patching the kernel.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>