net: usb: smsc: fix warning reported by kbuild test robot

This patch fixes following warning reported by kbuild test robot:

 In function ‘memcpy’,
     inlined from ‘smsc75xx_init_mac_address’ at drivers/net/usb/smsc75xx.c:778:3,
     inlined from ‘smsc75xx_bind’ at drivers/net/usb/smsc75xx.c:1501:2:
 ./include/linux/string.h:355:9: warning: argument 2 null where non-null expected [-Wnonnull]
   return __builtin_memcpy(p, q, size);
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
 drivers/net/usb/smsc75xx.c: In function ‘smsc75xx_bind’:
 ./include/linux/string.h:355:9: note: in a call to built-in function ‘__builtin_memcpy’

I've replaced the offending memcpy with ether_addr_copy, because I'm
100% sure, that of_get_mac_address can't return NULL as it returns valid
pointer or ERR_PTR encoded value, nothing else.

I'm hesitant to just change IS_ERR into IS_ERR_OR_NULL check, as this
would make the warning disappear also, but it would be confusing to
check for impossible return value just to make a compiler happy.

Fixes: adfb3cb2 ("net: usb: support of_get_mac_address new ERR_PTR error")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
Reviewed-by: Woojung Huh <woojung.huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>