[TG3]: Increase 5906 firmware poll time.

Newer 5906 bootcode needs about 7ms to finish resetting so the poll
firmware loop was changed to maximum 20ms.

Signed-off-by: Gary Zambrano <zambrano@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Acked-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: David S. Miller <davem@davemloft.net>