failover: Add missing check to validate 'slave_dev' in net_failover_slave_unregister

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/net_failover.c: In function 'net_failover_slave_unregister':
drivers/net/net_failover.c:598:35: warning:
 variable 'primary_dev' set but not used [-Wunused-but-set-variable]

There should check the validity of 'slave_dev'.

Fixes: cfc80d9a ("net: Introduce net_failover driver")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>