DSP: latch dma parameters

This needs to be tested a fair bit before it could be considered a candidate for merging.
DSP wasn't latching DMA parameters appropriately and it could break the audio output.

I don't have an extensive title collection, but if people could test this PR (windows binary: https://dl.dolphin-emu.org/prs/pr-692-dolphin-latest-x64.7z) and report any regressions, that would be awesome.
eg a nice way to check:

if (!g_audioDMA.AudioDMAControl.Enable)
{
    // check if this writes enables the dma
    g_audioDMA.AudioDMAControl.Hex = val;
    if (g_audioDMA.AudioDMAControl.Enable)
    {
        // TODO
    }
}
g_audioDMA.AudioDMAControl.Hex = val;