iwlwifi: mvm: combine part of the scan stop flows

For UMAC scans, we were simply jumping into another function when scan
stop functions were called, while for LMAC scans, the flow continued.
To make the flows cleaner and more balanced, combine the UMAC part
into the main stop functions.  This also makes us take one step closer
into combining the state flags for both APIs.

Note that some STOPPING flags will be dangling in UMAC scans, but it
doesn't matter because they are not used in UMAC yet (and this will be
fixed in subsequent patches).

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>