HID: wacom: put the managed resources in a group

We currently have a complex clean_inputs() function while this can be
handled all by devres. Set a group that we can destroy in wireless_work().

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