ath9k_htc: Handle pending URBs properly

commit ff8f59b5bbdf1527235b8c88d859c7d23691324f upstream.

When doing a channel change, the pending URBs have to be killed
properly on calling htc_stop().

This fixes the probe response timeout seen when sending UDP traffic at
a high rate and running background scan at the same time.

Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>