MLK-20589 sound:soc:codecs:ak4458: Don't mixed declarations and code

The patch fixes the following build warnings:

sound/soc/codecs/ak4458.c: In function ‘ak4458_hw_params’:
warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
  u8 dfs1, dfs2;
 ^~~

Signed-off-by: Jason Liu <jason.hui.liu@nxp.com>
CC: Shengjiu Wang <shengjiu.wang@nxp.com>
Acked-by: Shengjiu Wang <shengjiu.wang@nxp.com>