net: 3c574_cs fix stats.tx_bytes counter

Update the stats counter calculation in 3c574_cs, similar
to the method used in 3c589_cs. This corrects the contents
of the counter on tests using a "Megahertz 574B" card.

[linux@dominikbrodowski.net: clean up commit message]
Signed-off-by: Alexander Kurz <linux@kbdbabel.org>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: David S. Miller <davem@davemloft.net>