mac80211: fix CSA in IBSS mode

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>