mwifiex: set passive scan type for scan requests with no ssid

It was observed that station would sent probe request even when
scan type has been set as passive during iw scan.
This was happening because driver sets passive scan type only
when channel has IEEE80211_CHAN_NO_IR flag set.
Along with this, add condition to check if no ssids are specified in
scan request so as to mark such scan request passive.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>