dsound: Handle 8 channel audio input

[Philipp Middendorf]

Debugging why PES 2010 had sound effects but no music and no
commentator, I saw that the "cp_fields" function in 

dlls/dsound/mixer.c

was called with the parameters

device->pwfx->nChannels == 8 && dsb->pwfx->nChannels == 2

Since the case of 6 vs. 2 channels was already implemented and the code
didn't suggest that the number '6' was of much relevance, I added the
case 8 vs. 2 channels and PES 2010 worked perfectly. I quickly tested
it with another game, which also worked. The tests werde done on
an amd64 Gentoo Linux.