device/adsl: restore brfd value on error in br2684_assign_vcc()

Warned by coverity: we assert above that brfd is -1, so we must always
restore it to -1 in the error case.

Technically, not a problem because socket() is documented to return
only -1 on error already. Apparently coverity does not believe that.

(cherry picked from commit 5b9a848a82)

src/devices/adsl/nm-device-adsl.c

@@ -155,6 +155,7 @@ br2684_assign_vcc (NMDeviceAdsl *self, NMSettingAdsl *s_adsl)
 	if (priv->brfd < 0) {
 		errsv = errno;
 		_LOGE (LOGD_ADSL, "failed to open ATM control socket (%d)", errsv);
+		priv->brfd = -1;
 		return FALSE;
 	}