tpm_tis: limit size of buffer from backend

This is a preparatory patch for the subsequent ones where we
get rid of the flexibility of supporting any kind of buffer size
that the backend may support. We keep the size at 4096, which is
also the size the external emulator supports. So, limit the size
of the buffer we can support and pass it back to the backend.

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
