tpm_tis: move r/w_offsets to TPMState

Now that we have a single buffer, we also only need a single set of
read/write offsets into that buffer. This works since only one
locality can be active.

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