serial: 8250_of: Use of_device_get_match_data()

Use of_device_get_match_data() to simplify the code a bit.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: John Garry <john.garry@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>