ALSA: seq: Create device with snd_device_alloc()

Align with the other components, and use snd_device_alloc() for the
new sound device for sequencer, too.  No functional changes.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Curtis Malainey <cujomalainey@chromium.org>
Tested-by: Curtis Malainey <cujomalainey@chromium.org>
Link: https://lore.kernel.org/r/20230816160252.23396-9-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>