cros_ec: Don't return error when checking command version

With this patch, cros_ec_query_all() does not return an error if it
fails to check for MKBP events support. Instead, the EC device structure
indicates that it does not support MKBP events (mkbp_event_supported
field) and cros_ec_query_all() returns 0.

Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Signed-off-by: Benson Leung <bleung@chromium.org>