Change a couple of hardcoded numbers to PIO_INSTRUCTION_COUNT for clarity

[lurch]

32 is:

• number of words in a PIO's instruction memory
• number of GPIO pins that can be addressed by a PIO block
• number of bits that get shifted in or out by a PULL or PUSH instruction

so this PR clarifies "which 32" is being used here 😉 (Which IMHO also tallies nicely with the assert(program->length <= PIO_INSTRUCTION_COUNT); on line 67)

I was also tempted to change

if (program->origin > PIO_INSTRUCTION_COUNT - program->length) return PICO_ERROR_GENERIC;

to

if (program->origin > PIO_INSTRUCTION_COUNT - program->length) return PICO_ERROR_BAD_ALIGNMENT;

to match up with the error being returned here from add_program_at_offset_check, but I refrained from doing so in case that would be considered a backwards-incompatible change.