rt2x00: claim RSN IBSS support

Similar like other drivers, do not configure group keys to the hardware
(on Ad-Hoc mode) to make IBSS RSN work.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>