tg3: Show flowctrl settings through get_settings()

This patch adds code to present the flow control advertisements through
the ethtool get_settings callback.

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