stmmac: pci: Add dwmac support for Loongson

LoongArch inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5OHOB



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This GMAC module is integrated into the Loongson-2K SoC and the LS7A
bridge chip.

commit 30bba69d upstream.

Backport:
stmmac: dwmac-loongson: fix uninitialized variable in loongson_dwmac_probe()
stmmac: dwmac-loongson: Fix unsigned comparison to zero
stmmac: dwmac-loongson:Fix missing return value
stmmac: dwmac-loongson: change loongson_dwmac_driver from global to static
stmmac: pci: Add LS7A support for dwmac-loongson

This patch also disable dwmac FLOW_AUTO. Since DWMAC_LOONGSON do NOT support
FLOW_AUTO.

Signed-off-by: Qing Zhang <zhangqing@loongson.cn>
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ming Wang <wangming01@loongson.cn>
Change-Id: Ifefac7d47a05373ca7160d22a44d6c07b6a896e5