cxgb3 - Tighten xgmac workaround

Run the watchdog task when the link is up.
Flush the XGMAC Tx FIFO when the link drops.

Also remove a statistics update that should have gone
in the previous modification of xgmac.c.

Signed-off-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>