RCG: ADC read loop was only running once.

The loop was only running once for 65k IOP models with clock_div 2. Not enough values were written to ADC, starving user models.

Less obviously, Fast ADC were not being decimated at the right rate.

Standard IOPs or slow ADCs on a fast IOP were unaffected, since the loop is meant to run once.

closes #321 (closed)