So far, we reset the converter setups like the stream-tag, the
channel-id and format-id in prepare callbacks, and clear them in
cleanup callbacks.  This often causes a silence of the digital
receiver for a couple of seconds.

This patch tries to delay the converter setup changes as much as
possible.  The converter setups are cached and aren't reset as long
as the same values are used.  At suspend/resume, they are cleared
to be recovered properly, too.
Signed-off-by: Takashi Iwai <tiwai@suse.de>