tg3: Reduce 57765 core clock when link at 10Mbps

This patch reduces the core clock to 6.25MHz when operating at 10Mbps
link speed.  This is needed to prevent a bug that will ultimately cause
transmits to cease.

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