mkiss: Fix error handling in mkiss_open()

If register_netdev() fails we are not propagating the error and
we return success because ax_open() succeeded previously.

Fix this by checking the return value of ax_open() and
register_netdev() and propagate the error in case of failure.

Reported-by: RUC_Soft_Sec <zy900702@163.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>