i3c: fix an error code in i3c_master_add_i3c_dev_locked()

We should return "ret" as-is.  The "newdev" variable is a valid pointer.

Fixes: 3a379bbc ("i3c: Add core I3C infrastructure")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>