wmi: fix a memory leak in wmi_notify_debug

When acpi_evaluate_object() is passed ACPI_ALLOCATE_BUFFER,
the caller must kfree the returned buffer if AE_OK is returned.

The callers of wmi_get_event_data() pass ACPI_ALLOCATE_BUFFER,
and thus must check its return value before accessing
or kfree() on the buffer.

This patch adds return value checking for wmi_get_event_data()
and adds a missing kfree(obj) in the end of wmi_notify_debug

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Matthew Garrett <mjg@redhat.com>