staging: vt6656: rxtx.c Create union of struct vnt_rrv_time* stuctures

Attach union vnt_tx_head to structure vnt_tx_buffer replacing pointer
pvRrvTime.

In s_vGenerateTxParameter the relevant struct vnt_rrv_time_*
structure is attached to the vnt_tx_head.

The NULL check is now pFifoHead.

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