mac80211: Cancel the dynamic ps timer in ioctl_siwpower.

If the dynamic power save timer has been started before the power save
is disabled using iwconfig, we fail to cancel the timer. Hence cancel it
while disabling power save.

Signed-off-by: Vivek Natarajan <vnatarajan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>