platform/chrome: cros_ec_proto: return 0 on getting cmd mask success

cros_ec_get_host_command_version_mask() used to return value from
send_command() which is number of available bytes for input payload on
success (i.e. sizeof(struct ec_response_get_cmd_versions)).

However, the callers don't need to know how many bytes are available.

Don't return number of available bytes.  Instead, return 0 on success;
otherwise, negative integers on error.

Also remove the unneeded `ver_mask` initialization as the callers should
take it only if cros_ec_get_host_command_version_mask() returns 0.

Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20220609084957.3684698-15-tzungbi@kernel.org