tg3: Use mii_advertise_flowctrl

This patch replaces tg3's internal tg3_advert_flowctrl_1000T function
with mii_advertise_flowctrl provided by the kernel headers.

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@davemloft.net>