ibmvnic: correctly use dev_consume/free_skb_irq

It is more correct to use dev_kfree_skb_irq when packets are dropped,
and to use dev_consume_skb_irq when packets are consumed.

Fixes: 0d973388 ("ibmvnic: Introduce xmit_more support using batched subCRQ hcalls")
Suggested-by: Thomas Falcon <tlfalcon@linux.ibm.com>
Signed-off-by: Lijun Pan <lijunp213@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>