platform/chrome: cros_ec_typec: Make configure_mux static

Since cros_typec_configure_mux() is only used in cros-ec-typec,
it should be marked static.

Fixes: 7e7def15

 ("platform/chrome: cros_ec_typec: Add USB mux control")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Prashant Malani <pmalani@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>