ACPI: button: remove button->device pointer

We no longer need a pointer from struct acpi_button back to the
struct acpi_device.  Everywhere we used that pointer, we either
already have, or can easily get, the acpi_device pointer without
using the copy from acpi_button.  So this patch removes the
structure element.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>