ath9k: fix handling of rate control probe frames

The ath9k aggregation code was already checking the rate control probe flag
to prevent starting an aggregate frame with a sampling rate. What was missing
was closing an aggregate before adding a probing frame to it.
Without that, rate control cannot have precise control over probing, which
delays using faster rates when the channel conditions improve.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>