iwlwifi: mvm: reorganize scan stopping functions

The iwl_mvm_scan_offload_stop() function is used to stop LMAC regular
scan, stop LMAC scheduled scan and stop UMAC scheduled scans (but not
UMAC regular scans), making it very difficult to read.

Reorganize the scan stopping functions by creating separate functions
to stop regular and scheduled scans, separating the LMAC stopping part
of the code from the rest and renaming the offload_stop function to
iwl_mvm_lmac_scan_stop().

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