usb_audio: Report callback errors to syslog.

That way they don't just get lost.

usb_audio: Set real_time from the time during device buffers exchange.

As opposed to driver/userland buffers exchange. This is what the other
multi_audio drivers do.

usb_audio: Clean up buffer management.

* Get rid of fPacketSize and use fSamplesCount in GetBuffers instead.

* Clarify logic in _TransferCallback.

* Properly multiply in kSamplesBufferCount to fAreaSize
(this should be the only functional change in here.)

The larger buffers (now similarly sized to HDA at least in the
default cases of 44100 or 48000) seem to help with jittering
and discontinuities, which are significantly reduced now,
at least under emulation.

usb_audio: Yet more code cleanup.

Slight logic tweaks but no real behavioral changes intended.

usb_audio: Properly separate kernel and userland data.

* Allocate the descriptors array separately from the data area.
* Clone the data area for kernel use separately from userland use.

Fixes the remaining SMAP problems with this driver.

USB audio: Fix building on x86_64

* fix x86_64 build errors and warnings pointed out by Diver;
* fix warning building with gcc2.

Rename driver folder 'usb_audio' to 'usb'

... just because the source path ./kernel/drivers/audio/usb obviously
references audio hardware so "_audio" suffix is excessive.

No functional changes.

USB audio: Fix building on x86_64

* fix x86_64 build errors and warnings pointed out by Diver;
* fix warning building with gcc2.

Rename driver folder 'usb_audio' to 'usb'

... just because the source path ./kernel/drivers/audio/usb obviously
references audio hardware so "_audio" suffix is excessive.

No functional changes.