mwifiex: remove global user_scan_cfg variable

As the variable is used only for preparation of internal scan
commands, we don't need to keep it allocated until the entire
scan completes. We will define it as a local variable and free
immediately after it's use.

New flag 'scan_aborting' is added to handle race between
mwifiex_close() and scan handler. Previously user_scan_cfg
pointer used to take care of this.

This patch fixes a memory leak in mwifiex_cfg80211_scan after
running "iwlist mlan0 scan & sleep 1; rmmod mwifiex_sdio".

Reported-by: Daniel Drake <dsd@laptop.org>
Tested-by: Daniel Drake <dsd@laptop.org>
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>