rndis_wlan: workaround poor scanning with BCM4320a

BCM4320a devices seem to sometimes do scanning pretty poorly. This can be
workaround by issuing new scan every second, while not yet connected. By this
new scanning method device catches beacons much faster. Fixes bug #20822.

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