ao_coreaudio: remove volume multiplication by 4

kHALOutputParam_Volume is the linear gain so it should be at maximum 1 to
keep the audio quality good. No idea why it was more than that.
ao_coreaudio: remove chmap selection if format is digital
ao_coreaudio: move digital detection before asbd creation
ao_coreaudio: set channel layout bitmap
ao_coreaudio: only set chmap_sel info for lpcm
ao_coreaudio: remove some commented code

I left this in since I was unsure, but setting back samplerate and bps is
totally useless.
ao_coreaudio: remove useless function declaration
ao_coreaudio: return errors instead false in init functions
ao_coreaudio: remove TODO

FramesPerPacket is set later in intit_digital