mwifiex: do not use random MAC for pre-association scanning

Driver should use random MAC address only if the scan is
requested by user(provided NL80211_SCAN_FLAG_RANDOM_ADDR
is set in scan request). It should not be used for a scan
performed before association.

Signed-off-by: Ganapathi Bhat <gbhat@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>