ACPI / button: remove pointer to old lid_sysfs on unbind

When we removed the procfs dir on error or if the driver is
unbound, the two variables acpi_lid_dir and acpi_button_dir
were not reset. On the next rebind, those static variables
were not null and we couldn't re-register the device again.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/button.c

@@ -232,8 +232,10 @@ remove_dev_dir:
 	acpi_device_dir(device) = NULL;
 remove_lid_dir:
 	remove_proc_entry(ACPI_BUTTON_SUBCLASS_LID, acpi_button_dir);
+	acpi_lid_dir = NULL;
 remove_button_dir:
 	remove_proc_entry(ACPI_BUTTON_CLASS, acpi_root_dir);
+	acpi_button_dir = NULL;
 	goto done;
 }
 
@@ -250,7 +252,9 @@ static int acpi_button_remove_fs(struct acpi_device *device)
 			  acpi_lid_dir);
 	acpi_device_dir(device) = NULL;
 	remove_proc_entry(ACPI_BUTTON_SUBCLASS_LID, acpi_button_dir);
+	acpi_lid_dir = NULL;
 	remove_proc_entry(ACPI_BUTTON_CLASS, acpi_root_dir);
+	acpi_button_dir = NULL;
 
 	return 0;
 }