[TG3]: CPMU adjustments for loopback tests

This patch adds the LINK_SPEED mode to the list of CPMU modes that can
cause the loopback tests to fail.  These bugs are planned to be fixed in
future revisions of the chip, so the patch qualifies the fixes as such.

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>