HID: wacom: Constify attribute_groups

These are never modified, so make them const to allow the compiler to put
them in read-only memory. It also allows the compiler to shrink the
resulting module with ~900 bytes, test-built with gcc 10.2 on x86_64.

   text    data     bss     dec     hex filename
 204377   42832     576  247785   3c7e9 drivers/hid/wacom_old.ko
 204240   42064     576  246880   3c460 drivers/hid/wacom_new.ko

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>