rt2x00: Fix STBC transmissions to STAs with Rx STBC > 1

For STBC transmissions rt2x00 used the number of RxSTBC streams the
destination STA indicates in its HT capabilities as STBC value in
the TXWI.

However, the legacy drivers and our own comment in rt2800.h suggest
that the STBC field in the TXWI only allows a value of 0 or 1.
The values 2 and 3 are reserved (probably for future devices).

And indeed, STBC transmissions to STAs indicating more then 1
RxSTBC stream fail when the STBC field is set to something >1.

Fix this by only setting the STBC field to 1 when STBC should be
used.

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