net: stmmac: dwmac-loongson: Detach GMAC-specific platform data init

Loongson delivers two types of the network devices: Loongson GMAC and
Loongson GNET in the framework of four SOC/Chipsets revisions:

   Chip             Network  PCI Dev ID   Synopys Version   DMA-channel
LS2K1000 SOC         GMAC      0x7a03       v3.50a/v3.73a        1
LS7A1000 Chipset     GMAC      0x7a03       v3.50a/v3.73a        1
LS2K2000 SOC         GMAC      0x7a03          v3.73a            8
LS2K2000 SOC         GNET      0x7a13          v3.73a            8
LS7A2000 Chipset     GNET      0x7a13          v3.73a            1

The driver currently supports the chips with the Loongson GMAC network
device synthesized with a single DMA-channel available. As a
preparation before adding the Loongson GNET support detach the
Loongson GMAC-specific platform data initializations to the
loongson_gmac_data() method and preserve the common settings in the
loongson_default_data().

While at it drop the return value statement from the
loongson_default_data() method as redundant.

Note there is no intermediate vendor-specific PCS in between the MAC
and PHY on Loongson GMAC and GNET. So the plat->mac_interface field
can be freely initialized with the PHY_INTERFACE_MODE_NA value.

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>