Staging: vt6655: fix possible Read buffer overflow

If pDevice->sOpts.nRxDescs{0,1} or nTxDescs[{0,1}] is zero, the loop ends with
i == 0, and we write aRD{0,1}Ring[-1]. apTD{0,1}Rings[-1] respectively.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>