Fix build errors in Mediatek mtk_eth_soc driver.

It looks like these 3 source files were meant to be linked together
since 2 of them are library-like functions,
but they are currently being built as 3 loadable modules.

Fixes these build errors:

  WARNING: modpost: missing MODULE_LICENSE() in drivers/net/ethernet/mediatek/mtk_eth_path.o
  WARNING: modpost: missing MODULE_LICENSE() in drivers/net/ethernet/mediatek/mtk_sgmii.o
  ERROR: "mtk_sgmii_init" [drivers/net/ethernet/mediatek/mtk_eth_soc.ko] undefined!
  ERROR: "mtk_setup_hw_path" [drivers/net/ethernet/mediatek/mtk_eth_soc.ko] undefined!
  ERROR: "mtk_sgmii_setup_mode_force" [drivers/net/ethernet/mediatek/mtk_eth_soc.ko] undefined!
  ERROR: "mtk_sgmii_setup_mode_an" [drivers/net/ethernet/mediatek/mtk_eth_soc.ko] undefined!
  ERROR: "mtk_w32" [drivers/net/ethernet/mediatek/mtk_eth_path.ko] undefined!
  ERROR: "mtk_r32" [drivers/net/ethernet/mediatek/mtk_eth_path.ko] undefined!

This changes the loadable module name from mtk_eth_soc to mtk_eth.
I didn't see a way to leave it as mtk_eth_soc.
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Sean Wang <sean.wang@mediatek.com>
Cc: John Crispin <blogic@openwrt.org>
Cc: Felix Fietkau <nbd@openwrt.org>
Cc: Nelson Chang <nelson.chang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>