tpm: Restore functionality to xen vtpm driver.

Functionality of the xen-tpmfront driver was lost secondary to
the introduction of xenbus multi-page support in commit ccc9d90a
("xenbus_client: Extend interface to support multi-page ring").

In this commit pointer to location of where the shared page address
is stored was being passed to the xenbus_grant_ring() function rather
then the address of the shared page itself. This resulted in a situation
where the driver would attach to the vtpm-stubdom but any attempt
to send a command to the stub domain would timeout.

A diagnostic finding for this regression is the following error
message being generated when the xen-tpmfront driver probes for a
device:

<3>vtpm vtpm-0: tpm_transmit: tpm_send: error -62

<3>vtpm vtpm-0: A TPM error (-62) occurred attempting to determine
the timeouts

This fix is relevant to all kernels from 4.1 forward which is the
release in which multi-page xenbus support was introduced.

Daniel De Graaf formulated the fix by code inspection after the
regression point was located.

Fixes: ccc9d90a

 ("xenbus_client: Extend interface to support multi-page ring")
Signed-off-by: Dr. Greg Wettstein <greg@enjellic.com>

[boris: Updated commit message, added Fixes tag]
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: stable@vger.kernel.org # v4.1+
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>