ibmvnic: Clean actual number of RX or TX pools

Avoid using value stored in the login response buffer when
cleaning TX and RX buffer pools since these could be inconsistent
depending on the device state. Instead use the field in the driver's
private data that tracks the number of active pools.

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