mgmt: ec_host_cmd: improve handling buffer sizes

Add the len_max rx structure member to indicate maximum number of bytes
possible to receive. It is needed to send information about our protocol
parameters to host.

Also, limit the maximum size of request/responses for backends that uses
buffers provided by the handler.

Signed-off-by: Dawid Niedzwiecki <dawidn@google.com>
7 files changed