mac80211: stop queues before rate control updation

Stop tx queues before updating rate control to ensure
proper rate selection. Otherwise packets can be transmitted
in 40 Mhz whereas hw is configured in HT20.

Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>