sandbox: cros_ec: exynos: Drop use of cros_ec_get_error()

This function is really just a call to uclass_get_device() and there is no
reason why the caller cannot do it. Update sandbox and snow accordingly.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Minkyu Kang <mk7.kang@samsung.com>