staging: vt6656: rxtx.c parse out struct vnt_tx_datahead_a_fb

Parse struct vnt_tx_datahead_a_fb from s_uFillDataHead and
move to struct vnt_rts_a_fb structure returning duration down
functions.

This removes pvTxDataHd pointer assignment.

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