mac80211: Update device channel in case of HW channel switch supported

The hw.conf.channel value is not updated properly for drivers that
support HW channel switch.  Since the switch is done entirely by the
driver and we don't call ieee80211_hw_config(), this value remains
untouched.  This patch fixes that by setting the new channel directly in
ieee80211_chswitch_work().

Signed-off-by: Shahar Levi <shahar_levi@ti.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>