[TG3]: Add 1000T & 1000X flowctl adv helpers

This patch adds two functions designed to convert abstract TX & RX
flow control parameters to 1000-BaseT and 1000-BaseX autonegotiation
advertisements.  Code that uses standard definitions which statically
advertises TX & RX flow control has been replaced with code that
configures the advertisements based on administrator dictated
preferences.

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>