ALSA: dummy: Fix trailing whitespaces.

Fix checkpatch.pl ERROR: trailing whitespaces.
Signed-off-by: YJ Lee <yunjunlee@chromium.org>
Link: https://lore.kernel.org/r/20220912072854.760824-1-yunjunlee@chromium.orgSigned-off-by: Takashi Iwai <tiwai@suse.de>