mwifiex: cleanup work in scan.c

Scan type derived from IEEE80211_CHAN_PASSIVE_SCAN bit is a boolean
flag representing passive scanning. We should not again compare it
with driver specific macro MWIFIEX_SCAN_TYPE_PASSIVE to determine
passive or active scan. We can also avoid the use of local variable
by using the flag directly.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>