sim: sh: fix uninitialized variable usage with pdmsb

sim: sh: fix uninitialized variable usage with pdmsb

This block of code relies on i to control which bits to test and how
many times to run through the loop, but it never actually initialized
it.  There is another chunk of code that handles the pdmsb instruction
that sets i to 16, so use that here too assuming it's correct.  The
programming manual suggests this is the right value too, but I am by
no means a SuperH DSP expert.  The tests are still passing though ...