USB: mxuport: fix null deref when used as a console

Fix null-pointer dereference at probe when the device is used as a
console, in which case the tty argument to open will be NULL.

Fixes: ee467a1f ("USB: serial: add Moxa UPORT 12XX/14XX/16XX
driver")
Cc: stable <stable@vger.kernel.org>	# v3.14
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Greg Kroah-Hartman <greg@kroah.com>