Starting audio or seeking in various music players causes
setup_dig_out_stream() to be called, which resets the SPDIF stream,
which caused one DAC (but not another) to make a clicking noise every
time.

This patch ensures the reset only happens when it needs to, which is
when the format changes, and makes the code a little more readable.
Signed-off-by: NLaurence Darby <ldarby@tuffmail.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>