e1000: fix wrong queue idx calculation

tx_ring and adapter->tx_ring are already of type "struct
e1000_tx_ring *"

Signed-off-by: Hong Zhiguo <zhiguohong@tencent.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>