mediatek: fix compile error caused by upstreamed mtk_m32 function

mediatek: fix compile error caused by upstreamed mtk_m32 function

This fixes following compile error:

```
  CC      drivers/net/ethernet/mediatek/mtk_eth_soc.o
drivers/net/ethernet/mediatek/mtk_eth_soc.c:70:5: error: conflicting types for 'mtk_m32'
 u32 mtk_m32(struct mtk_eth *eth, u32 mask, u32 set, unsigned reg)
     ^~~~~~~
In file included from drivers/net/ethernet/mediatek/mtk_eth_soc.c:25:
drivers/net/ethernet/mediatek/mtk_eth_soc.h:964:6: note: previous declaration of 'mtk_m32' was here
 void mtk_m32(struct mtk_eth *eth, u32 clear, u32 set, unsigned reg);
      ^~~~~~~
```

which caused by 5.4.34 mainlining of mtk_m32 function.

Fixes: faf668be354a ("kernel: bump 5.4 to 5.4.34")
Signed-off-by: Yong-hyu, Ban <perillamint@quendi.moe>