Currently the registration and free of beep input device was done
manually from the register and the disconnect callbacks of the
assigned codec object.  This seems working in most cases, but this may
be a cause of some races at probe.  Moreover, due to these manual
calls, the total code became unnecessarily lengthy.

This patch rewrites the beep registration code to follow the standard
sound device object style.  This allows us reducing the code, in
addition to avoiding the nested device registration calls.
Signed-off-by: Takashi Iwai <tiwai@suse.de>