platform/chrome: cros_ec_proto: update cros_ec_check_result() comment

At first glance, cros_ec_check_result() is quite like cros_ec_map_error().
They check for `ec_msg->result` and return corresponding errors.  However,
as calling from `pkt_xfer` and `cmd_xfer`, cros_ec_check_result() should
not report furthermore errors.  -EAGAIN is the only exception.

See [1][2][3] for some known userland programs' code.  The return code
from ioctl only denotes the EC communication status.  Userland programs
would further analyze the `result` in struct cros_ec_command* for
follow-up actions (e.g. [4]).

To clarify, update the function comment.

[1]: https://crrev.com/54400e93a75ef440a83d6eaac2cec066daf99cf0/util/comm-dev.c#154
[2]: https://crrev.com/fe32670a89bf59e1aff84bba9dd3295657b85e9b/cros_ec_dev.c#296
[3]: https://crrev.com/4e19eb1d89de0422ff1bbd3f7260b131c761098c/drivers/google/cros_ec_dev.c#120
[4]: https://crrev.com/54400e93a75ef440a83d6eaac2cec066daf99cf0/util/comm-dev.c#164



Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Link: https://lore.kernel.org/r/20220518091814.2028579-4-tzungbi@kernel.org