mwifiex: avoid storing random_mac in private

Application will keep track of whether MAC address randomization
is enabled or not during scan. But at present driver is storing
'random_mac' in mwifiex_private which implies even after scan is
done driver has some reference to the earlier 'scan request'. To
avoid this, make use of 'mac_addr' variable in 'scan_request' to
store 'random_mac'. This structure will be freed by cfg80211 once
scan is done.

Signed-off-by: Ganapathi Bhat <gbhat@marvell.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>