HID: wiimote: convert KEYS and RUMBLE to modules

This introduces the first sub-device modules by converting the KEYS and
RUMBLE sub-devices into wiimote modules. Both must be converted at once
because they depend on the built-in shared input device.

This mostly moves code from wiimote-core to wiimote-modules and doesn't
change any semantics or ABI.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>