tg3: Fix 5714S / 5715S / 5780S link failures

The git commit ef167e27 entitled
"Fix supporting flowctrl code" introduced a bug that prevents 5714S,
5715S and 5780S devices from falling back to a forced link mode.  The
problem is that the added flow control check will always fail if flow
control is set to autoneg and either RX or TX (or both) flow control
is enabled.  The driver defaults to setting flow control to autoneg
and advertises both RX and TX flow control.

The fix is to remove the errant check.

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>