mac80211: minstrel: do not sample rates 3 times slower than max_prob_rate

These rates are highly unlikely to be used quickly, even if the link
deteriorates rapidly. This improves throughput in cases where CCK rates
are not reliable enough to be skipped entirely during sampling.
Sampling these rates regularly can cost a lot of airtime.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>