tg3: Add TG3_FLG3_USE_PHYLIB

This patch introduces the TG3_FLG3_USE_PHYLIB flag and applies it to
some select places.  This work makes later patches a little easier to
read.

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