[TG3]: 5784 / 5764 DMA engine lockup fix

5784 and 5764 devices lock up when the link speed is 10Mbps, the CPMU
link speed mode is enabled, and the MAC clock is running at 1.5Mhz.  The
fix is to run the MAC clock at faster speeds.

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>