ALSA: ac97: fix build regression

The ac97_bus_type structure is no longer declared in this file:

sound/ac97/bus.c: In function 'ac97_codec_add':
sound/ac97/bus.c:112:27: error: 'ac97_bus_type' undeclared (first use in this function); did you mean 'bus_type'?
  112 |         codec->dev.bus = &ac97_bus_type;
      |                           ^~~~~~~~~~~~~
      |                           bus_type
sound/ac97/bus.c:112:27: note: each undeclared identifier is reported only once for each function it appears in
sound/ac97/bus.c: In function 'snd_ac97_codec_driver_register':
sound/ac97/bus.c:191:28: error: 'ac97_bus_type' undeclared (first use in this function); did you mean 'ac97_bus_reset'?
  191 |         drv->driver.bus = &ac97_bus_type;

Include the header that contains the declaration and make sure the definition
is const but not static.

Fixes: 66e82d21

 ("ALSA: mark all struct bus_type as const")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20240103102544.3715055-1-arnd@kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>