cros_ec_cmd_xfer() is the only exported function that calls static
function cros_ec_send_command().

Add Kunit tests for cros_ec_send_command() through calling
cros_ec_cmd_xfer().
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20220718050914.2267370-5-tzungbi@kernel.org