Fix "ubi part" cmd re-entrancy

Commit 2ee951ba (UBI: Enable re-initializing of the "ubi part" command)
reset mtd_devs in ubi_exit() but missed ubi_init()'s failure path.

Signed-off-by: Karl Beldan <karl.beldan@gmail.com>
Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>

drivers/mtd/ubi/build.c

@@ -1045,6 +1045,7 @@ out_version:
 out_class:
 	class_destroy(ubi_class);
 out:
+	mtd_devs = 0;
 	ubi_err("UBI error: cannot initialize UBI, error %d", err);
 	return err;
 }