net_failover: fixed rollback in net_failover_open()

found by smatch:
drivers/net/net_failover.c:65 net_failover_open() error:
 we previously assumed 'primary_dev' could be null (see line 43)

Fixes: cfc80d9a ("net: Introduce net_failover driver")
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>