udev: fix review mixup

The previous version in the PR changed variable and sanitized it
in place. The second version switched to skip if CCs are in the
string instead, but didn't move back to the original variable.
Because it's an existing variable, no CI caught it.

Follow-up for 16325b35fa

src/udev/scsi_id/scsi_id.c

@@ -443,7 +443,7 @@ static int scsi_id(char *maj_min_dev) {
                 if (dev_scsi.tgpt_group[0] != '\0')
                         printf("ID_TARGET_PORT=%s\n", dev_scsi.tgpt_group);
                 if (dev_scsi.unit_serial_number[0] != '\0' && utf8_is_valid(dev_scsi.unit_serial_number) && !string_has_cc(dev_scsi.unit_serial_number, /* ok= */ NULL))
-                        printf("ID_SCSI_SERIAL=%s\n", serial_str);
+                        printf("ID_SCSI_SERIAL=%s\n", dev_scsi.unit_serial_number);
                 goto out;
         }