sound/usbsoundbasedevice: Unregister device removed handler

The device removed handler for the underlying USB audio streaming device
must be unregistered, when the class CUSBSoundBaseDevice is destroyed.
Otherwise it may be called with an invalid this-pointer, when the
streaming device is removed, which led to a system crash.

The device remove handling for the underlying streaming device has been
simplified. There is only one registration of the removed handler any
more. The removed handler waits for all USB data transfers to complete
and the sound device going idle now.

Before, multiple instances of CUSBSoundBaseDevice tried to register the
same device name "sndusb", which failed from the second instance on,
because the name was already in use. Now the second instance gets the
name "sndusb1" and so on.