Fix to return a negative error code from the failover register fail
error handling case instead of 0, as done elsewhere in this function.

Fixes: 1ff78076 ("netvsc: refactor notifier/event handling code to use the failover framework")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>