tg3: Consolidate autoneg advertisement setup code

Autonegotiation setup has gotten a little more complicated since the tg3
driver was created.  This patch consolidates autoneg setup into one
routine and modifies the call sites accordingly.

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