cfg80211: fix wdev tracing crash

Arend reported a crash in tracing if the driver returns an
ERR_PTR() value from the add_virtual_intf() callback. This
is due to the tracing then still attempting to dereference
the "pointer", fix this by using IS_ERR_OR_NULL().
Reported-by: Arend van Spriel <arend@broadcom.com>
Tested-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/trace.h

@@ -27,7 +27,8 @@
 #define WIPHY_PR_ARG	__entry->wiphy_name
 
 #define WDEV_ENTRY	__field(u32, id)
-#define WDEV_ASSIGN	(__entry->id) = (wdev ? wdev->identifier : 0)
+#define WDEV_ASSIGN	(__entry->id) = (!IS_ERR_OR_NULL(wdev)	\
+					 ? wdev->identifier : 0)
 #define WDEV_PR_FMT	"wdev(%u)"
 #define WDEV_PR_ARG	(__entry->id)