ibmvnic: Call napi_disable instead of napi_enable in failure path

The failure path in ibmvnic_open() mistakenly makes a second call
to napi_enable instead of calling napi_disable. This can result
in a BUG_ON for any queues that were enabled in the previous call
to napi_enable.

Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>