USB: wacom tablet driver reorganization

- split wacom.c into 4 files: wacom.h, wacom_wac.h, wacom_sys.c, and wacom_wac.c
- where wacom_sys.c deals with system specific code, 
- and wacom_wac.c deals with Wacom specific code

Signed-off-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>