ASoC: fixup oops in generic AC97 codec glue

Initialize the glue by calling snd_soc_new_ac97_codec() as is done
in other ASoC AC97 codecs.  Fixes an oops caused by dereferencing
uninitialized members in snd_soc_new_pcms().

Run-tested on Au1250.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>