[netdevice] Increase maximum network-layer address length

IPv6 uses 16-byte network-layer addresses.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/netdevice.h

 #define MAX_LL_HEADER_LEN 36
 
 /** Maximum length of a network-layer address */
-#define MAX_NET_ADDR_LEN 4
+#define MAX_NET_ADDR_LEN 16
 
 /** Maximum length of a network-layer header
  *