mac80211/minstrel_ht: fix spacing between sample attempts

A sample attempt should only count in mi->sample_tries if the sample
attempt wasn't skipped based on slower rate criteria.
This patch increases the sampling frequency for potentially desirable
rates and thus enables faster recovery from interference or collisions.

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