If driver_register() failed there is no sense to call driver_unregister().
unregister_chrdev() should be called here.

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>