HID: wacom: Refactor button-to-key translation into function

This just centralizes the logic used in both wacom_setup_numbered_buttons
and wacom_report_numbered_buttons so that they don't drift out of sync.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>