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

To new structure.
typedef struct tagSTxDataHead_a_FB -> struct vnt_tx_datahead_a_fb

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>