serdev: ttyport: check whether tty_init_dev() fails

My static checker complains that we don't have any error handling here.
It's simple enough to add it.

Fixes: bed35c6d ("serdev: add a tty port controller driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>