Input: wacom - fix TabletPC touch bug

This patch fixed a bug that was introduced in kernel 2.6.28 for
TabletPC touch data. The wacom_parse_hid routine in wacom_sys.c
should always return 0 even when usb_control_msg got an error.

Signed-off-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>