[TG3]: Correct 5704S flowctrl advertisements

This patch modifies the 5704S hardware autoneg code to use the
administrator specified flow control parameters.  Since the 5704S uses
device specific flow control enumerations, the 1000-BaseX utility
functions are used and code was added to convert the definitions to and
from the proprietary enumerations.

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>