Update headerguards
Remove obsolete struct madifx_mixer

Replaced by struct madifx_newmixer. To distinguish leftover code from
new code, the new mixer is called newmixer. ;)
Drop old mixer support
Only compile mixer and level interface if SND_MADIFX_BROKEN is set.

For kernel mainline inclusion, we disable all unstable bits of the userspace
API.

For the standalone driver, we'll set the define to 1.
Conditionally enable CONFIG_SND_MADIFX_BROKEN

If you want experimental/broken WIP features, compile with

    make BROKEN_WIP=1
Mention BROKEN_WIP=1 in the README
Disable all references to old mixer.

The OLD_MIXER define enables/disables old HDSPM mixer code. Since this
code is known not to work for MADIFX, it doesn't make sense to enable
it. It's only there for future reference and might come in handy when
hacking the new MADIFX-specific mixer support.
Fix comment about mixer interna.
[cosmetics] Trim comment