tg3: Fix ethtool loopback test for 5761 BX devices

A CPMU related loopback test bug existed for AX revisions of the 5761.
While that errata has been fixed, the CPMU still slows down the core
clock too far to run the loopback test successfully.  This patch
disables the CPMU LINK_SPEED mode just like we do with the AX
revisions of the 5761 and all revisions of the 5784.

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>