staging: vt6656: rxtx.c parse out vnt_rxtx_datahead_ab

Parse struct vnt_tx_datahead_ab from s_uFillDataHead and
move to struct vnt_rts_ab structure returning duration down
functions.

Both are returned in s_uFillDataHead at this stage as pvTxDataHd
is only null in s_bPacketToWirelessUsb.

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