tg3: Prevent a PCIe tx glitch

This patch prevents a PCIe tx glitch by allowing the transmitter to go
to a low power state.

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