mac80211: don't transmit 40MHz frames to 20MHz peer

If a mesh peer indicates it is operating as 20MHz-only in its HT
operation IE, have the rate control algorithm respect this by disabling
the equivalent bit in the ieee80211_sta HT capabilities.

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