uio: fix memory leak

we have to call kobject_put() to clean up the kobject after function
kobject_init(), kobject_add(), or kobject_uevent() is called.

Signed-off-by: Cong Ding <dinggnu@gmail.com>
Acked-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>