platform/chrome: cros_ec_proto: return -EAGAIN when retries timed out

While EC_COMMS_STATUS_PROCESSING flag is still on after it tries
EC_COMMAND_RETRIES times for sending EC_CMD_GET_COMMS_STATUS,
cros_ec_wait_until_complete() doesn't return an error code.

Return -EAGAIN in the case instead.

Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20220718050914.2267370-9-tzungbi@kernel.org