UBI: fix return code in ubi_volume_read

Return -ENODEV instead of 0 when trying to read from a non existing volume.

Signed-off-by: Andreas Huber <andreas.huber@keymile.com>
Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/common/cmd_ubi.c b/common/cmd_ubi.c
index 02a2e55..64a7307 100644
--- a/common/cmd_ubi.c
+++ b/common/cmd_ubi.c
@@ -327,7 +327,7 @@
 	}
 	if (i == ubi->vtbl_slots) {
 		printf("%s volume not found\n", volume);
-		return 0;
+		return -ENODEV;
 	}
 
 	printf("read %i bytes from volume %d to %x(buf address)\n",