rt2x00: rt2800: disable TX STBC for 1 stream devices

Disable TX STBC for 1 stream devices as a minimum of 2 streams is needed for TX STBC.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>