platform/x86: dell-wmi: Convert to the WMI bus infrastructure

Move some initialization out of _init and into _probe.
Update signatures and logic to use the wmi bus and device structures.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
[dvhart: drop deprecated sparse_keymap_free, order declarations, add commit msg]
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Mario Limonciello <mario_limonciello@dell.com>
Cc: Pali Rohár <pali.rohar@gmail.com>
Cc: linux-kernel@vger.kernel.org
Cc: platform-driver-x86@vger.kernel.org
Cc: linux-acpi@vger.kernel.org
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>