ibmvnic: Deactivate RX pool buffer replenishment on H_CLOSED

If H_CLOSED is returned, halt RX buffer replenishment activity
until firmware sends a notification that the driver can reset.

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