tg3: Assign correct tx margin for 5719

Commit d309a46e42542223946d3a9e4e239fdc945cb53e, entitled
"tg3: 5719: Prevent tx data corruption", was supposed to contain the tx
margin adjustment but it looks like it somehow was omitted.  This patch
fixes the problem.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>