serial: 8250: dw: Fix NULL pointer dereference

dw8250_platform_data is only used on DT platforms for now.

Fixes: 4a218b27 ("serial: 8250: dw: Create a generic platform data structure")
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220502115621.77985-1-heikki.krogerus@linux.intel.com


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