tg3: Fix disappearing 57780 devices

Under certain power saving conditions, 57780 asic rev devices might
disappear from the system.  The fix is to disallow the PCIe PLL from
powering down.

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