ath5k: fix missing output in monitor mode after ifconfig up

Let ath5k_chan_set() always call ath5k_reset().
This fixes the bug that we don't
get any packets in monitor mode after:

ifconfig wlan0 down
iwconfig wlan0 mode monitor channel 1
ifconfig wlan0 up

but they arrive after

iwconfig wlan0 channel 2

Signed-off-by: Joerg Albert <jal2@gmx.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>