mac80211: minstrel: fix using short preamble CCK rates on HT clients

[ Upstream commit 37439f2d6e43ae79e22be9be159f0af157468f82 ]

mi->supported[MINSTREL_CCK_GROUP] needs to be updated
short preamble rates need to be marked as supported regardless of
whether it's currently enabled. Its state can change at any time without
a rate_update call.

Fixes: 782dda00 ("mac80211: minstrel_ht: move short preamble check out of get_rate")
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>