platform/chrome: cros_ec_ishtp: Fix a double-unlock issue

In function cros_ec_ishtp_probe(), "up_write" is already called
before function "cros_ec_dev_init". But "up_write" will be called
again after the calling of the function "cros_ec_dev_init" failed.
Thus add a call of the function “down_write” in this if branch
for the completion of the exception handling.

Fixes: 26a14267 ("platform/chrome: Add ChromeOS EC ISHTP driver")
Signed-off-by: Qiushi Wu <wu000273@umn.edu>
Tested-by: Mathew King <mathewk@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>