iwlwifi: mvm: rename generic_scan_cmd functions to dwell

The generic scan command functions are now irrelevant, since both
sched and regular scans are in the same code.  So rename this
functions to dwell and isolate all the dwell-related setting to them.
Keeping the dwell code separate makes it easier to compare the LMAC
and UMAC versions.

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