ibmvnic: Revert to previous mtu when unsupported value requested

If we request an unsupported mtu value, the vnic server will suggest a
different value. Currently we take the suggested value without question
and login with that value. However, the behavior doesn't seem completely
sane as attempting to change the mtu to some specific value will change
the mtu to some completely different value most of the time. This patch
fixes the issue by logging in with the previously used mtu value and
printing an error message saying that the given mtu is unsupported.

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