HID: wacom: Move WAC_CMD_* into wacom_wac.h

Centralize our definition of report IDs by moving those for device commands
into wacom_wac.h alongside those for input reports.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>