Functions won't be directly used outside of compress.c file
so mark them as static.

This will also fix warnings reported by kernel test robot:

>> sound/soc/sof/compress.c:91:5: warning: no previous prototype for
function 'sof_compr_open' [-Wmissing-prototypes]
   int sof_compr_open(struct snd_soc_component *component,
       ^
   sound/soc/sof/compress.c:91:1: note: declare 'static' if the function
is not intended to be used outside of this translation unit
   int sof_compr_open(struct snd_soc_component *component,

Fixes: 6324cf90 ("SoC: SOF: compr: Add compress ops implementation")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://lore.kernel.org/r/20220211082631.179735-1-daniel.baluta@oss.nxp.comSigned-off-by: Mark Brown <broonie@kernel.org>