mac80211: Allow deleting stations in ibss mode to reset their state

Set the NL80211_EXT_FEATURE_DEL_IBSS_STA if the interface support IBSS
mode, so that stations can be reset from user space.

mac80211 already deletes stations by itself, so mac80211 drivers must
already support this.

This has been successfully tested with ath9k.

Signed-off-by: Nicolas Cavallari <nicolas.cavallari@green-communications.fr>
Link: https://lore.kernel.org/r/20200305135754.12094-2-cavallar@lri.fr
Signed-off-by: Johannes Berg <johannes.berg@intel.com>