staging: vt6656: desc.h remove typedef tagSTxDataHead_ab to new structure in rxtx.h

To new structure.
typedef struct tagSTxDataHead_ab -> struct vnt_tx_datahead_ab

This is only needed by rxtc.c so moved to rxtx.h visible to
vnt_rts* structures which it will eventually form part of
their structure.

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