iwlwifi: mvm: check if scan can be started before cancelling other scans

If a new scan cannot be run for some reason, we shouldn't cancel other
ongoing scans.  Move the checks to before the code that cancels other
scans.

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>