rndis_wlan: enable stall workaround by link quality instead of link speed

Enable workaround for poor link stalls by link quality instead of link
speed. Using link speed caused workaround be active always on 802.11b
networks which reduced performance and not even catch all stalls.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>