audio: Clean up inappropriate and unreachable use of hw_error()

audio_init() should not use hw_error(), because dumping CPU registers
is unhelpful there, and aborting is wrong, because it can be called
called from an audio device's realize() method.

The two uses of hw_error() come from commit 0d9acba:

* When qemu_new_timer() fails.  It couldn't fail back then, and it
  can't fail now.  Drop the unreachable error handling.

* When no_audio_driver can't be initialized.  It couldn't fail back
  then, and it can't fail now.  Replace the error handling by an
  assertion.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Markus Armbruster <armbru@pond.sub.org>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
