ramips: 6.1: ralink: fix const warning in the ethernet driver

ramips: 6.1: ralink: fix const warning in the ethernet driver

Change fe_hw_set_macaddr and the set_mac parameter to const to fix
errors in the form of:

   drivers/net/ethernet/ralink/mtk_eth_soc.c: In function 'fe_set_mac_address':
  drivers/net/ethernet/ralink/mtk_eth_soc.c:174:53: error: passing argument 2 of 'priv->soc->set_mac' discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]
    174 |                         priv->soc->set_mac(priv, dev->dev_addr);
        |                                                  ~~~^~~~~~~~~~
  drivers/net/ethernet/ralink/mtk_eth_soc.c:174:53: note: expected 'unsigned char *' but argument is of type 'const unsigned char *'
  drivers/net/ethernet/ralink/mtk_eth_soc.c: In function 'fe_hw_init':
  drivers/net/ethernet/ralink/mtk_eth_soc.c:1220:45: error: passing argument 2 of 'priv->soc->set_mac' discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]
   1220 |                 priv->soc->set_mac(priv, dev->dev_addr);
        |                                          ~~~^~~~~~~~~~
  drivers/net/ethernet/ralink/mtk_eth_soc.c:1220:45: note: expected 'unsigned char *' but argument is of type 'const unsigned char *'
  drivers/net/ethernet/ralink/mtk_eth_soc.c:1222:44: error: passing argument 2 of 'fe_hw_set_macaddr' discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]
   1222 |                 fe_hw_set_macaddr(priv, dev->dev_addr);
        |                                         ~~~^~~~~~~~~~
  drivers/net/ethernet/ralink/mtk_eth_soc.c:155:75: note: expected 'unsigned char *' but argument is of type 'const unsigned char *'
    155 | static inline void fe_hw_set_macaddr(struct fe_priv *priv, unsigned char *mac)
        |                                                            ~~~~~~~~~~~~~~~^~~
  cc1: all warnings being treated as errors

Signed-off-by: Nick Hainke <vincent@systemli.org>