mac80211: fix STA channel width field

According to IEEE 802.11 8.4.2.59, set the "STA channel width" bit to 0
if transmitting STA is using a 20mhz channel.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>