staging: et131x: improve code consistency: access struct tx_ring

Let all the code use a pointer called tx_ring to access struct tx_ring
members.

Signed-off-by: ZHAO Gang <gamerh2o@gmail.com>
Acked-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>