nl80211: does not allow NEW_STATION and DEL_STATION for mesh

As discussed in linux-wireless mailing list, adding and removing
stations for mesh topologies is not necessary. Since doing it triggers
bugs, the sugestion was to simply disable it.

Tested using a custom iw command "station new". Works only after using
hostapd. "station del" command also works.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: Simon Raffeiner <sturmflut@lieberbiber.de>
Cc: Andrey Yurovsky <andrey@cozybit.com>
Cc: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>