HID: wacom: EKR: use devres groups to manage resources

This will be useful when each remote will be assigned its own input device.
We won't need to unregister each input and sysfs and other elements one
at a time.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>