ASoC: soc-cache: Remove unneeded codec_drv pointer variable in snd_soc_flat_cache_init

Since commit d779fce5
"ASoC: soc-cache: Ensure flat compression uses a copy of the defaults cache",
the codec_drv pointer variable is not used any more.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>