netsniff-ng: fix snooping on any device when no option is given

Commit f43bbe9 ("mac80211: Check existence of generated monX
device") broke starting netsniff-ng w/o any arguments, that is,
sniffing on "any" device. The test in device_ifindex() should be
index < 0.

Fixes: f43bbe9 ("mac80211: Check existence of generated monX device")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

dev.c

@@ -39,7 +39,8 @@ int device_ifindex_get(const char *ifname)
 int device_ifindex(const char *ifname)
 {
 	int index = device_ifindex_get(ifname);
-	if (unlikely(index <= 0))
+
+	if (unlikely(index < 0))
 		panic("Cannot get ifindex from device!\n");
 
 	return index;