platform/chrome: cros_ec_proto: handle empty payload in getting wake mask

cros_ec_get_host_event_wake_mask() expects to receive
sizeof(struct ec_response_host_event_mask) from send_command().  The
payload is valid only if the return value is positive.

Return -EPROTO if send_command() returns 0 in
cros_ec_get_host_event_wake_mask().

Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20220609084957.3684698-22-tzungbi@kernel.org