tty_port: make tty_port_register_device wrap tty_port_register_device_attr

tty_register_device is just a wrapper for tty_register_device_attr with
NULL passed for drvdata and attr_grp. So similarly make
tty_port_register_device a wrapper of tty_port_register_device_attr so that
additions don't have to be made in both functions.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-By: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>