stmmac: pci: split out common_default_data() helper

New helper is added in order to prevent misconfiguration happened
for one of the platforms when configuration data is expanded.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Joao Pinto <jpinto@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>