Function SilcSFTPDataCallback
SYNOPSIS
typedef void (*SilcSFTPDataCallback)(SilcSFTP sftp,
SilcSFTPStatus status,
const unsigned char *data,
uint32 data_len,
void *context);
DESCRIPTION
Data callback is called when data packet is received from the server.
This is called for example when application is reading a file from
the server. The `data' is the raw data of length of `data_len'.