ibmvnic: Fix login buffer memory leaks

During device bringup, the driver exchanges login buffers with
firmware. These buffers contain information such number of TX
and RX queues alloted to the device, RX buffer size, etc. These
buffers weren't being properly freed on device reset or close.

We can free the buffer we send to firmware as soon as we get
a response. There is information in the response buffer that
the driver needs for normal operation so retain it until the
next reset or removal.

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