ACPI: configfs: Add missing config_item_put() to fix refcount leak

config_item_put() should be called in the drop_item callback, to
decrement refcount for the config item.

Fixes: 772bf1e2 ("ACPI: configfs: Unload SSDT on configfs entry removal")
Signed-off-by: Hanjun Guo <guohanjun@huawei.com>
[ rjw: Subject edit ]
Cc: 4.13+ <stable@vger.kernel.org> # 4.13+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>