This adds support for Wacom Stylus device with I2C interface.

[Dan Carpenter <dan.carpenter@oracle.com>: fix NULL-pointer dereference
 in error handling path.]
Signed-off-by: Tatsunosuke Tobita <tobita.tatsunosuke@wacom.co.jp>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>