ideapad_laptop: introduce struct acpi_device pointer to ideapad_private structure

Introduce struct acpi_device pointer to ideapad_private structure.
At the same time, replace all adevice with adev to be short and consistent.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
CC: Matthew Garrett <matthew.garrett@nebula.com>
CC: Ike Panhc <ike.pan@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>