net: stmmac: dwmac-loongson: Use PCI_DEVICE_DATA() macro for device identification

For the readability sake convert the hard-coded Loongson GMAC PCI ID to
the respective macro and use the PCI_DEVICE_DATA() macro-function to
create the pci_device_id array entry. The later change will be
specifically useful in order to assign the device-specific data for the
currently supported device and for about to be added Loongson GNET
controller.

Signed-off-by: Feiyang Chen <chenfeiyang@loongson.cn>
Signed-off-by: Yinggang Gu <guyinggang@loongson.cn>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Acked-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Tested-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>