Fix style according to kernel coding standards

This commit fixes all ERRORs and some warnings reported by checkpatch.

No functional change.
Change location of io.h include

Reported by checkpatch
Remove unused leftover

This was a leftover from the HDSPM driver.
Remove leftover prototype declaration

Yet another leftover from the HDSPM driver
Remove leftover all_in_all mixer

No longer needed, functionality moved to snd_madifx_create().
Make mixer WIP and compile only on demand.
Remove warning about unused variable