mac80211: minstrel: fix sampling/reporting of CCK rates in HT mode

[ Upstream commit 972b66b86f85f4e8201db454f4c3e9d990cf9836 ]

Long/short preamble selection cannot be sampled separately, since it
depends on the BSS state. Because of that, sampling attempts to
currently not used preamble modes are not counted in the statistics,
which leads to CCK rates being sampled too often.

Fix statistics accounting for long/short preamble by increasing the
index where necessary.
Fix excessive CCK rate sampling by dropping unsupported sample attempts.

This improves throughput on 2.4 GHz channels

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>