ibmvnic: Wait for device response when changing MAC

Wait for a response from the VNIC server before exiting after setting
the MAC address. The resolves an issue with bonding a VNIC client in
ALB or TLB modes. The bonding driver was changing the MAC address more
rapidly than the device could respond, causing the following errors.

"bond0: the hw address of slave eth2 is in use by the bond;
couldn't find a slave with a free hw address to give it
(this should not have happened)"

If the function waits until the change is finalized, these errors are
avoided.

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