sound: usb: Send explicit device disconnect to clients

Explicitly send device disconnect notification to client
so that client can disable audio streams. This will in turn
enable sound usb driver to release usb device and notify device
removal to user-space. User-space will clean up audio session.
Without this change sound usb driver is blocking usb device
release till audio streams are disabled. However, audio streams
are only disabled when user-space cleans up audio session on usb
device removal which is blocked. This change is trying to fix
this deadlock condition by notifying device disconnect to clients
explicitly.

Change-Id: I7e5a8aca84a7a620bb61eb0ace8a6b6c622f89de
Signed-off-by: Vamsi Krishna Samavedam <vskrishn@codeaurora.org>