ALSA: pcm: oss: Use guard() for setup

The setup_mutex in PCM oss code can be simplified with guard().
(params_lock is tough and not trivial to covert, though.)

Only the code refactoring, and no functional changes.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20240227085306.9764-24-tiwai@suse.de