tg3: Move EEE definitions into mdio.h

In commit 52b02d04 entitled "tg3: Add
EEE support", Ben Hutchings had commented that the EEE advertisement
register will be in a standard location.  This patch moves that
definition into mdio.h and changes the code to use it.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>