cardscan: fix [-Wunused-but-set-variable] compiler warning

backend/cardscan.c

@@ -874,7 +874,6 @@ sane_control_option (SANE_Handle handle, SANE_Int option,
   }
   else if (action == SANE_ACTION_SET_VALUE) {
       int tmp;
-      SANE_Word val_c;
       SANE_Status status;
 
       DBG (20, "sane_control_option: set value for '%s' (%d)\n", s->opt[option].name,option);
@@ -895,9 +894,6 @@ sane_control_option (SANE_Handle handle, SANE_Int option,
         return status;
       }
 
-      /* may have been changed by constrain, so dont copy until now */
-      val_c = *(SANE_Word *)val;
-
       /*
        * Note - for those options which can assume one of a list of
        * valid values, we can safely assume that they will have