ASoC: Intel: bytcr_rt5640: Mark hp_elitepad_1000g2_jack?_check functions static

The byt_rt5640_hp_elitepad_1000g2_jack?_check functions are only
used inside bytcr_rt5640.c, mark them as static.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20210823110432.64860-1-hdegoede@redhat.com
Signed-off-by: Mark Brown <broonie@kernel.org>