iwlwifi: mvm: fix incorrect fallthrough in iwl_mvm_check_running_scans()

In the iwl_mvm_check_running_scans() we were mistakenly ignoring the
value returned by iwl_mvm_scan_stop() for scheduled scans and falling
thorugh to the next case, which caused us to always return zero.

Signed-off-by: Nicholas Krause <xerofoify@gmail.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>