b43legacy: Fix to enhance TX speed

Recent changes in the specifications have improved the performance
of the BCM4306/2 devices that use b43legacy as the driver. These
"errors" in the specs have been present from the very first implementation
of bcm43xx.

Signed-off-by: Ehud Gavron <gavron@wetwork.net>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>