tpm_tis: remove TPMSizeBuffer usage

Remove usage of TPMSizeBuffer. The size of the buffers is limited now
by s->be_buffer_size, which is the size of the buffer the TIS has
negotiated with the backend.

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