wl1271: Disable host TX rate control

Disable host TX rate control. The wl1271 firmware is already managing rate
control, so this eliminate unnecessary host processing.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>