staging: vt6656: rxtx.c: s_vGetFreeContext use single tx memset.

memset to 0 structure _USB_SEND_CONTEXT pContext->Data to its
size of MAX_TOTAL_SIZE_WITH_ALL_HEADERS as defined
in device.h

This removes the need for memset in the TX path.

It also allows the removal of dead structure
TX_BUFFER_CONTAINER BufferHeader;

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>