[ipv6] Do not set sin6_scope_id on source address

The transmitting network device is specified via the destination
address, not the source address.  There is no reason to set
sin6_scope_id on the source address.

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

src/net/ndp.c

 	sin6_src.sin6_family = AF_INET6;
 	memcpy ( &sin6_src.sin6_addr, net_source,
 		 sizeof ( sin6_src.sin6_addr ) );
-	sin6_src.sin6_scope_id = netdev->index;
 
 	/* Construct multicast destination address */
 	memset ( &sin6_dest, 0, sizeof ( sin6_dest ) );