net: nixge: Address compiler warnings about signedness

Fixes the following warnings:
warning: pointer targets in passing argument 1 of
‘is_valid_ether_addr’ differ in signedness [-Wpointer-sign]
  if (mac_addr && is_valid_ether_addr(mac_addr)) {
                                      ^~~~~~~~
expected ‘const u8 * {aka const unsigned char *}’ but argument
is of type ‘const char *’
 static inline bool is_valid_ether_addr(const u8 *addr)
                    ^~~~~~~~~~~~~~~~~~~
warning: pointer targets in passing argument 2 of
‘ether_addr_copy’ differ in signedness [-Wpointer-sign]
   ether_addr_copy(ndev->dev_addr, mac_addr);
                                   ^~~~~~~~
expected ‘const u8 * {aka const unsigned char *}’ but argument
is of type ‘const char *’
 static inline void ether_addr_copy(u8 *dst, const u8 *src)

Signed-off-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/ni/nixge.c

@@ -1183,7 +1183,7 @@ static int nixge_probe(struct platform_device *pdev)
 	struct nixge_priv *priv;
 	struct net_device *ndev;
 	struct resource *dmares;
-	const char *mac_addr;
+	const u8 *mac_addr;
 	int err;
 
 	ndev = alloc_etherdev(sizeof(*priv));