minstrel_ht: increase sampling frequency

Try to sample all available rates, as sample attempts do not cost much
airtime and are appropriately spaced based on the average A-MPDU length.
This helps with faster recovery on rate fluctuations.

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