rt2x00: Restructure bw_comp calculation

Move the HT40 check inside the calculation function to make it easier
for a later cleanup.

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