HID: wacom: EKR: have proper allocator and destructor

The wacom_remote_create_attr_group() and wacom_remote_destroy_attr_group()
functions were both allocating/destroying the sysfs groups but also
initializing the parameters for the remotes. Have proper functions
that can be called and extended.

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