mac80211: minstrel: fix CCK rate group streams value

[ Upstream commit 80df9be67c44cb636bbc92caeddad8caf334c53c ]

Fixes a harmless underflow issue when CCK rates are actively being used

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>