acpi_ec: clean up 'private' ivar when freeing memory to which it points

This is not only a prudent thing to do, but also makes sure that probe
method is not confused by non-NULL 'private', if the previous attach
attempt fails for any reason.

PR:		kern/142561
Tested by:	Alex Goncharov <alex-goncharov@comcast.net>
MFC after:	4 days

sys/dev/acpica/acpi_ec.c

@@ -469,6 +469,7 @@ acpi_ec_attach(device_t dev)
     sc->ec_gpehandle = params->gpe_handle;
     sc->ec_uid = params->uid;
     sc->ec_suspending = FALSE;
+    acpi_set_private(dev, NULL);
     free(params, M_TEMP);
 
     /* Attach bus resources for data and command/status ports. */