staging: bcm2835-audio: Remove redundant spdif stream ctls

The "IEC958 Playback Stream" control does basically the very same
thing as "IEC958 Playback Default" redundantly.  The former should
have been stream-specific and restored after closing the stream, but
we don't do in that way.

Since it's nothing but confusion, remove this fake.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>