ibmvnic: Ensure that TX queues are disabled in __ibmvnic_close

Use netif_tx_disable to guarantee that TX queues are disabled
when __ibmvnic_close is called by the device reset routine.

Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>