bugfix: instead of rate-limiting the DSP time constant value directly,
maintain the original value written and rate-limit a copy on DMA start.
This fixes a problem with sample rate limits and Optic Nerve demo. The
demo appears to first test the SB by using DSP command 0x14 to play a
2-byte long block, then issue DSP command 0x91 for the main demo music
without writing the time constant again between the two.

Prior to this fix, the DSP time constant was clamped down to non-hispeed
22050Hz and then not restored for the 0x91 playback, causing the demo
music to play slowly.