serial: 8250_exar: Extract exar_get_platform() helper

We would like to use DMI matching in other functions as well.
Hence, extract it as exar_get_platform() helper function.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210608144239.12697-2-andriy.shevchenko@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>