HID: wacom: Clean up use of struct->wacom_wac

Replace the indirect accesses to struct->wacom_wac from struct->wacom
to the direct access in order for better code reading.

Signed-off-by: Tatsunosuke Tobita <tatsunosuke.tobita@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>