wireless: disable wext sysfs by default

This code has been on the list to remove for a long
time, so disable it by default, add a warning to its
Kconfig, and schedule it for removal in 3.5.

The only known dependency, hal, has not required it
since its 0.5.12 release, which was in early 2009
and hal has since been deprecated completely.

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