net: wilc1000: clean up resource in error path of init mon interface

The wilc_wfi_init_mon_int() forgets to clean up resource when
register_netdevice() failed. Add the missed call to fix it.
And the return value of netdev_priv can't be NULL, so remove
the unnecessary error handling.

Fixes: 588713006e ("staging: wilc1000: avoid the use of 'wilc_wfi_mon' static variable")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Huang Guobin <huangguobin4@huawei.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200917123019.206382-1-huangguobin4@huawei.com

drivers/net/wireless/microchip/wilc1000/mon.c

@@ -235,11 +235,10 @@ struct net_device *wilc_wfi_init_mon_interface(struct wilc *wl,
 
 	if (register_netdevice(wl->monitor_dev)) {
 		netdev_err(real_dev, "register_netdevice failed\n");
+		free_netdev(wl->monitor_dev);
 		return NULL;
 	}
 	priv = netdev_priv(wl->monitor_dev);
-	if (!priv)
-		return NULL;
 
 	priv->real_ndev = real_dev;