greybus: hid: fix null-deref on short report requests

Make sure to verify the length of incoming requests before trying to
parse the request buffer, which can even be NULL on empty requests.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>