[S390] dasd: fix return code handling

Check return value of itcw_add_dcw() for error code and return it.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/block/dasd_eckd.c

@@ -2552,8 +2552,7 @@ static int prepare_itcw(struct itcw *itcw,
 
 	dcw = itcw_add_dcw(itcw, pfx_cmd, 0,
 		     &pfxdata, sizeof(pfxdata), total_data_size);
-
-	return rc;
+	return IS_ERR(dcw) ? PTR_ERR(dcw) : 0;
 }
 
 static struct dasd_ccw_req *dasd_eckd_build_cp_tpm_track(