Add the missing IBSS capability flag during capability init as it needs
to be inserted into the generated beacon in order for CSA to work.

Fixes: cd7760e6 ("mac80211: add support for CSA in IBSS mode")
Signed-off-by: Piotr Gawlowicz <gawlowicz@tkn.tu-berlin.de>
Signed-off-by: Mikołaj Chwalisz <chwalisz@tkn.tu-berlin.de>
Tested-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>