brcmfmac: Check null pointer on brcmf_dev_reset.

When unloading it is possible that drvr is not null, but iflist[0]
is. So check iflist[0] pointer before using it.
Reviewed-by: NArend Van Spriel <arend@broadcom.com>
Reviewed-by: NPieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: NPiotr Haber <phaber@broadcom.com>
Signed-off-by: NHante Meuleman <meuleman@broadcom.com>
Signed-off-by: NArend van Spriel <arend@broadcom.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

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

@@ -842,6 +842,7 @@ void brcmf_dev_reset(struct device *dev)
 	if (drvr == NULL)
 		return;
 
+	if (drvr->iflist[0])
 		brcmf_fil_cmd_int_set(drvr->iflist[0], BRCMF_C_TERMINATED, 1);
 }