usb: xhci: Clean up error code in xhci_dbc_tty_register_device()

tty_port_register_device() returns error pointers on error, never NULL.
The IS_ERR_OR_NULL() function returns either 1 or 0 so it means we
return 1 on error instead of a proper error code.  The caller only
checks for zero vs non-zero so this doesn't affect runtime.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>