Sound Blaster 16 PnP emulation: Added code to enforce constraints noted
on an actual SB16 ViBRA test card, where the 16-bit DMA channel is
permitted to be either 5, 6, or 7, or match the 8-bit DMA, but if you
configure the 16-bit DMA on any other channel (like, say, 8-bit DMA on
channel 1 and 16-bit DMA on channel 3) the card doesn't seem to cycle
DMA at all.