i2c: cros-ec-tunnel: Fix usage of cros_ec_cmd_xfer()

cros_ec_cmd_xfer returns success status if the command transport
completes successfully, but the execution result is incorrectly ignored.
In many cases, the execution result is assumed to be successful, leading
to ignored errors and operating on uninitialized data.

We've recently introduced the cros_ec_cmd_xfer_status() helper to avoid these
problems. Let's use it.

[Regarding the 'Fixes' tag; there is significant refactoring since the driver's
introduction, but the underlying logical error exists throughout I believe]

Fixes: 9d230c9e ("i2c: ChromeOS EC tunnel driver")
Cc: <stable@vger.kernel.org> # 9798ac6d

 mfd: cros_ec: Add cros_ec_cmd_xfer_status() helper
Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>