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>