[ALSA] remove redundent assignment to the ac97 device structure

AC97 Codec
Don't use dev.platform_data to store a reference to the containing
ac97_t structure.  Such assignment is redundent since we can deduce the
ac97_t structure location from the contained device structure.  This
sets platform_data free for other purposes.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>