Input: wacom - references to 'wacom->data' should use 'unsigned char*'

'wacom->data' contains raw binary data and can lead to unexpected behavior
if a byte under examination happens to have its MSB set.

Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Tested-by: Aaron Skomra <Aaron.Skomra@wacom.com>
Reviewed-by: Carl Worth <cworth@cworth.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>