iwlwifi: mvm: take scan ref only on success

In some cases, scan reference was taken, but wasn't
released even though scan command wasn't actually issued.

Change the current code to simply take the reference
only on success.

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>