ibmvnic: Correct ibmvnic handling of device open/close

When closing the ibmvnic device we need to release the resources used
in communicating to the virtual I/O server. These need to be
re-negotiated with the server at open time.

This patch moves the releasing of resources a separate routine
and updates the open and close handlers to release all resources at
close and re-negotiate and allocate these resources at open.

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