Use C99 struct init for struct audio_hw_if

Use C99 struct init for struct audio_hw_if

This audio(9) struct will lose a member, but drivers init their struct quite
inconsistently, most pre-C99 style.

Use C99 style everywhere, to get rid of all annoying differences and allow
for easy member removals/additions:
- don't change current order of members
- no explicit NULL members
- no comments or blank lines
- trailing comma in last member line

GENERIC.MP builds fine with this on arm64, amd64, i386 and sparc64.

macppc and alpha build-tested by miod
OK ratchov miod