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>