tpm xen: drop unneeded chip variable

The call that used chip was dropped in 1f0f30e4.  Drop the
leftover declaration and initialization.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>