sound: swarm_cs4297: Provide definitions for AC97 registers

This patch removes the last usage of linux/ac97_codec.h by
re-defining used AC97 registers.
We can't use sound/ac97_codec.h here, since it is an OSS driver.

Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>