net: ag71xx: fix discards 'const' qualifier warning

Current kernel will compile this driver with warnings. This patch will
fix it.

drivers/net/ethernet/atheros/ag71xx.c: In function 'ag71xx_fast_reset':
drivers/net/ethernet/atheros/ag71xx.c:996:31: warning: passing argument 2 of 'ag71xx_hw_set
_macaddr' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  996 |  ag71xx_hw_set_macaddr(ag, dev->dev_addr);
      |                            ~~~^~~~~~~~~~
drivers/net/ethernet/atheros/ag71xx.c:951:69: note: expected 'unsigned char *' but argument
 is of type 'const unsigned char *'
  951 | static void ag71xx_hw_set_macaddr(struct ag71xx *ag, unsigned char *mac)
      |                                                      ~~~~~~~~~~~~~~~^~~
drivers/net/ethernet/atheros/ag71xx.c: In function 'ag71xx_open':
drivers/net/ethernet/atheros/ag71xx.c:1441:32: warning: passing argument 2 of 'ag71xx_hw_se
t_macaddr' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
 1441 |  ag71xx_hw_set_macaddr(ag, ndev->dev_addr);
      |                            ~~~~^~~~~~~~~~
drivers/net/ethernet/atheros/ag71xx.c:951:69: note: expected 'unsigned char *' but argument
 is of type 'const unsigned char *'
  951 | static void ag71xx_hw_set_macaddr(struct ag71xx *ag, unsigned char *mac)
      |                                                      ~~~~~~~~~~~~~~~^~~

Fixes: adeef3e3 ("net: constify netdev->dev_addr")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>