igb: Use rx/tx_itr_setting when setting up initial value of itr

It turns out that the InterruptThrottleRate module parameter was only
having the effect of locking the ITR at the starting ITR value. This was
because the values stored in rx_itr_setting and tx_itr_setting were being
ignored when configuring the initial itr_val of the q_vector.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>