rt2x00: Only initialize HT on rt2800 devices that support it.

Some RT28xx/RT30xx devices don't support 802.11n, when they are combined with
the RF2020 chipset. Ensure that HT is disabled for these devices.

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