tg3: Prevent tx BD corruption

tg3: Prevent tx BD corruption

This patch prevents a tx BD corruption bug by preventing the device from
powering down the PLL from L1 if the link speed is 10Mbps or 100Mbps.

The same bits are also used to prevent a system hang during chip reset
resulting from a complicated set of events that ultimately leads to
PCIe block register corruption.

Signed-off-by: Matt Carlson <[email protected]>
Reviewed-by: Michael Chan <[email protected]>
Reviewed-by: Benjamin Li <[email protected]>
Signed-off-by: David S. Miller <[email protected]>