iwlwifi: mvm: rename umac scan stop function

For consistency with the LMAC functions, rename the UMAC scan stop
function to iwl_mvm_umac_scan_stop().  Additionally, move things
around a bit to avoid an unnecessary forward declaration.

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