staging: vt6656: desc.h remove typedef SRrvTime_ab to new structure.

To new structure.
typedef struct tagSRrvTime_ab -> struct vnt_rrv_time_ab

This is only needed by rxtc.c so moved to rxtx.h and
will eventually form part of the structure of
struct vnt_tx_buffer.

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