rt2x00: rt2800lib: disable HT40 for now as it causes reception problems

Disable HT40 support for now as it causes rx problems with HT40 capable
11n APs (when mac80211 enables HT40, rx is completely disfunctional).

Once the rt2800 HT code is capable of using HT40 we should enable the
flag again.

I only tested this patch with a rt305x SoC device, nevertheless the
patch disables HT40 also on PCI and USB rt2800 devices.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>