platform/chrome: cros_ec_proto: don't show MKBP version if unsupported

It wrongly showed the following message when it doesn't support MKBP:
"MKBP support version 4294967295".

Fix it.

Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20220609084957.3684698-14-tzungbi@kernel.org