disk/part_dos.c: Fix a variable typo in write_mbr_partitions()

This function is passed *dev not *dev_desc, so pass the right name to
part_init().

Fixes: f14c5ee5ab8b ("disk: part_dos: update partition table entries after write")
Signed-off-by: Christian Melki <christian.melki@t2data.com>
diff --git a/disk/part_dos.c b/disk/part_dos.c
index 60addc6..9e29aa6 100644
--- a/disk/part_dos.c
+++ b/disk/part_dos.c
@@ -424,7 +424,7 @@
 	}
 
 	/* Update the partition table entries*/
-	part_init(dev_desc);
+	part_init(dev);
 
 	return 0;
 }