via-rhine: close SMP transmit races.

7ab87ff4 ("via-rhine: move work from
irq handler to softirq and beyond") forgot to explicitely control the
lifespan of the tx_dirty and tx_cur pointers.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>