RDMA/ocrdma: Remove a unused-label warning

If IPV6 is disabled, we get the warning:

    drivers/infiniband/hw/ocrdma/ocrdma_main.c:650:1: warning: label ‘err_notifier6’ defined but not used [-Wunused-label]

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>