tg3: Prevent corruption at 10 / 100Mbps w CLKREQ

This patch disables CLKREQ at 10Mbps and 100Mbps to workaround a TX BD
corruption issue.  This problem only affects the 5784 and 5761 (and
57780 AX) ASIC revisions.

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>