cros_ec_prepare_tx() returns either:
- >= 0 for number of prepared bytes.
- < 0 for -errno.

Correct the comment and make sure all callers check the return code.
Reviewed-by: NGuenter Roeck <groeck@chromium.org>
Signed-off-by: NTzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20220513044143.1045728-3-tzungbi@kernel.org