mac80211: update minstrel_ht sample rate when probe is set

Waiting until the status is received can cause the same rate to be
probed multiple times consecutively.

Cc: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Daniel Halperin <dhalperi@cs.washington.edu>
Signed-off-by: John W. Linville <linville@tuxdriver.com>