forcedeth: fix tx limit2 flag check

This is a fix for bug 572201 @ bugs.debian.org

This patch fixes the TX_LIMIT feature flag. The previous logic check
for TX_LIMIT2 also took into account a device that only had TX_LIMIT
set.

Reported-by: Stephen Mulcahu <stephen.mulcahy@deri.org>
Reported-by: Ben Huchings <ben@decadent.org.uk>
Signed-off-by: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>