tpm_tis: merge r/w_offset into rw_offset

We can now merge the r_offset and w_offset into a single rw_offset.
This is possible since when the offset is used for writing in
RECEPTION state then reads are ignore. Conversely, when the offset
is used for reading when in COMPLETION state, then writes are
ignored.

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