brcmfmac: fix NULL pointer access in brcmf_fweh_detach()

brcmf_fweh_detach can be called while ifp is already NULL, due to
init error. Fix NULL pointer access by checking ifp.
Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/brcm80211/brcmfmac/fweh.c

@@ -374,11 +374,13 @@ void brcmf_fweh_detach(struct brcmf_pub *drvr)
 	struct brcmf_if *ifp = drvr->iflist[0];
 	s8 eventmask[BRCMF_EVENTING_MASK_LEN];
 
+	if (ifp) {
 		/* clear all events */
 		memset(eventmask, 0, BRCMF_EVENTING_MASK_LEN);
 		(void)brcmf_fil_iovar_data_set(ifp, "event_msgs",
-				       eventmask, BRCMF_EVENTING_MASK_LEN);
-
+					       eventmask,
+					       BRCMF_EVENTING_MASK_LEN);
+	}
 	/* cancel the worker */
 	cancel_work_sync(&fweh->event_work);
 	WARN_ON(!list_empty(&fweh->event_q));