iwlwifi: queue user-initiated scan when doing internal scan

The internal scanning created a problem where
when userspace tries to scan, the scan gets
rejected. Instead of doing that, queue up the
user-initiated scan when doing an internal
scan.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>