printf-hook-builtin: Correctly calculate written bytes in print_in_hook()

printf-hook-builtin: Correctly calculate written bytes in print_in_hook()

The hook data counts remaining buffer bytes, not used ones. Counting them
correctly fixes a crash for long hexdumps.

Further, print_in_hook() must return the number of bytes that would have been
written, not the actually written bytes. This is important, as we allocate a
dynamic buffer in bus that relies on the exact byte count. Fixes long hexdumps
that got truncated.