rndis_wlan: add support for set_cqm_rssi_config

Device poller already reads current RSSI, so add support for
set_cqm_rssi_config there.

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