HID: wacom: EKR: allocate one input node per remote

Thanks to devres, we can now afford to create more than one input node
without having to overload the remove/failure paths. Having one input
node per remote is something which should have been implemented from start
but the probability of having users with several remotes is quite low.
Anyway, still, better looking at the future and implement things properly.

Remote input nodes will be freed/unregistered magically as they are
created in the devres group &remote->remotes[index].

We need to open the hid node now that the remotes are dynamically
allocated.

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