sparc64: ensure VIO operations are defined while being used

It's possible that VIO operations are not defined for some VIO
clients. In that case, VIO ops pointer should be checked for
NULL before being used

Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Reviewed-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>