serial: 8250_dw: add separate pointer for the uart_port to dw8250_probe

For convenience, adding separate pointer for the "port"
member of struct uart_8250_port that is being filled in the
probe function.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>