iwlwifi: fix unloading driver while scanning

If NetworkManager is busy scanning when user
tries to unload the module, the driver can not be unloaded
because HW still scanning.

Make sure driver sends abort scan host command to uCode if it
is in the middle of scanning during driver unload.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>