cxgb3: Update Rev3 mac workaround

Update the heurstics workaround unlocking a hung mac:
- reduce Tx mac toggling by enabling Tx drain before resetting the mac
- Take Tx (lack of) activity in account only
- Update the monitoring counter range to 64 bits

Signed-off-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>