mac80211: fix an issue in ieee80211_tx_info count field management

I noticed a possible issue in the status count field management of the
ieee80211_tx_info data structure. In particular, when the AGGR
processing is employed,
status.rates[].count is set just for the first frame and not for
others belonging to the same burst, leading to wrong statistic data in
the mac80211 debug file system.

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>