ALSA: dummy: Convert to generic PCM copy ops

This patch converts the dummy driver code to use the new unified PCM
copy callback.  As dummy driver doesn't do anything in the callback,
it's just a simple replacement.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230815190136.8987-5-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>