[TG3]: Add 1000T & 1000X flowctrl resolvers

This patch adds two new utility functions to resolve flow control.  One
function resolves flow control based on 1000-BaseT register definitions.
The other resolves flow control based on 1000-Base X register
definitions.

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>