rt2x00: Fix HT40+/HT40- setting in rt2800.

Inspection of the Ralink vendor driver shows that the TX_BAND_CFG register
and BBP register 3 are about HT40- indication, not about HT40+ indication.
Inverse the meaning of these fields in the code.

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