ASoC: remove include of pxa2xx-pcm.h in pxa2xx-ac97.c

Fix reference to moved header file, which was unused anyway.
This change fixes below build error:
  CC      sound/soc/pxa/pxa2xx-ac97.o
sound/soc/pxa/pxa2xx-ac97.c:27:24: error: pxa2xx-pcm.h: No such file or directory
make[3]: *** [sound/soc/pxa/pxa2xx-ac97.o] Error 1
make[2]: *** [sound/soc/pxa] Error 2
make[1]: *** [sound/soc] Error 2
make: *** [sound] Error 2

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>