ACPI: button: cache hid/name/class pointers

This patch adds temporaries to cache the acpi_device_hid(),
acpi_device_name(), and acpi_device_class() pointers so we
don't have to clutter the code with so many uses of those
interfaces.

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