rndis_wlan: get bssid scan list before new scan

OID_802_11_BSSID_LIST_SCAN clears device's bssid list, so retrieve
current bssid list from device before issuing new scan.

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