ath9k_htc: cancel ani work in ath9k_htc_stop

ani work is cancelled in dissaoctiation. But in some cases
during suspend, deauthention never be called. So we failed
to stop ani work which was identified by the following
warning.

Call Trace:
 [<ffffffffa0454a1d>] ieee80211_can_queue_work.clone.17+0x2d/0x40 [mac80211]
 [<ffffffffa0454a60>] ieee80211_queue_delayed_work+0x30/0x60 [mac80211]
 [<ffffffffa0567f82>] ath9k_ani_work+0x142/0x250 [ath9k_htc]
 [<ffffffff81073c70>] async_run_entry_fn+0x0/0x180
 [<ffffffffa0567e40>] ath9k_ani_work+0x0/0x250 [ath9k_htc]

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