HID: wacom: Fix several minor compiler warnings

Addresses a few issues that were noticed when compiling with non-default
warnings enabled. The trimmed-down warnings in the order they are fixed
below are:

* declaration of 'size' shadows a parameter

* '%s' directive output may be truncated writing up to 5 bytes into a
  region of size between 1 and 64

* pointer targets in initialization of 'char *' from 'unsigned char *'
  differ in signedness

* left shift of negative value

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Aaron Armstrong Skomra <aaron.skomra@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>