ibmvnic: Exit polling routine correctly during adapter reset

This patch fixes a bug where, in the case of a device reset,
the polling routine will never complete, causing napi_disable
to sleep indefinitely when attempting to close the device.

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