i40evf: add missing kernel-doc argument

@flush has been missing since the inception of i40evf_irq_enable(). Add
it for the kernel doc.
Signed-off-by: NJean Sacren <sakiwit@gmail.com>
Tested-by: NAndrew Bowers <andrewx.bowers@intel.com>
Acked-by: NJesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/i40evf/i40evf_main.c

@@ -282,6 +282,7 @@ static void i40evf_fire_sw_int(struct i40evf_adapter *adapter, u32 mask)
 /**
  * i40evf_irq_enable - Enable default interrupt generation settings
  * @adapter: board private structure
+ * @flush: boolean value whether to run rd32()
  **/
 void i40evf_irq_enable(struct i40evf_adapter *adapter, bool flush)
 {