tg3: Revise 5719 internal FIFO overflow solution

Commit cf79003d, entitled
"tg3: Fix 5719 internal FIFO overflow problem", proposed a way to solve
an internal FIFO overflow problem.  We have since discovered a slightly
better way to solve the problem.  This patch changes the code so that
the problem is contained closer to the problem source.

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