platform/chrome: cros_ec_ishtp: Make init_lock static

The init_lock is not declared or used outside of cros_ec_ishtp.c
so make it static to avoid the following warning:

drivers/platform/chrome/cros_ec_ishtp.c:79:1: warning: symbol 'init_lock' was not declared. Should it be static?

Signed-off-by: Ben Dooks (Codethink) <ben.dooks@codethink.co.uk>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>