iwlwifi: mvm: convert scan_status to a bitmap

LMAC scans cannot handle more than one scan at a time, but UMAC scans
can.  To avoid confusion we should combine the states of these two
types of scans.  To do so, we need to support mutliple scans at the
same time for UMAC.

This commit changes the scan_status element from a single value to a
bitmask of running scan types for LMAC.  Later, we will modify UMAC
scans to use the same state bitmask.

Additionally, add stopping scan flags for scheduled and regular scans.
This makes it easier to differentiate and handle stop requests
triggered by the driver and spontaneous stops generated by the
firmware.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>