via-cuda: Initialize data_index early and increment consistently

Initialize data_index where appropriate to improve readability and
assist debugging. This change doesn't affect driver behaviour.

I prefer to see
	current_req->data[data_index++]
in place of
	current_req->data[0]
or
	current_req->data[1]
inasmuchas it becomes obvious what the data_index variable does.

Moreover, the actual value of data_index when examined at any given moment
tells me something about prior events, which did prove helpful.

Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>