ASoC: max98388: set variable soc_codec_dev_max98388 storage-class-specifier to static

smatch reports
sound/soc/codecs/max98388.c:890:39: warning: symbol
  'soc_codec_dev_max98388' was not declared. Should it be static?

This variable is only used in its defining file, so it should be static.

Signed-off-by: Tom Rix <trix@redhat.com>
Link: https://lore.kernel.org/r/Message-Id

: <20230617122635.3225639-1-trix@redhat.com>
Signed-off-by: Mark Brown <broonie@kernel.org>