wext: remove a/b/g/n from SIOCGIWNAME

Since a/b/g/n no longer exist as spec amendements and VHT (ex 802.11ac)
wasn't handled at all, it's better to just remove the amendment strings
to avoid confusion.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>