igb: Fix NULL assignment to incorrect variable in igb_reset_q_vector

adapter->tx_ring is set to NULL where rx_ring should be.

Fixes: 5536d210

 ("igb: Combine q_vector and ring allocation into a single function")
Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>