carl9170: claim to support IBSS RSN.

On carl9170, HW encryption is disabled on IBSS; the mac80211
software-based encryption is used instead. As mac80211 supports IBSS
RSN (per-STA GTK), claim its support in the carl9170 PHY.

Signed-off-by: Nicolas Cavallari <cavallar@lri.fr>
Acked-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>