mac80211/minstrel_ht: disable multiple consecutive sample attempts

The last minstrel_ht changes increased the sampling frequency for
potentially useful rates to decrease the response time to rate
fluctuations. This caused an increase in sampling frequency that can
slightly reduce throughput, so this patch limits the sampling attempts
to one per rate instead of two.

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