tg3: Convert code to use PHY_IS_FET

This patch converts the code to use the PHY_IS_FET flag rather than the
ASIC revision to decide whether or not to use FET paths.

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