[TG3]: Separate requested and actual flow control parameters

This patch removes the TX and RX flow control flags from tg3_flags and
adds two new flow control variables, flowctrl and active_flowctrl.

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>