tg3: Prevent send BD corruption

On rare occasions, send BD corruptions can occur.  This patch
fixes the problem by increasing the L1 entry threshold to 4
milliseconds.

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