TPM: ReadPubEK output struct fix

This patch adds a missing element of the ReadPubEK command output,
that prevents future overflow of this buffer when copying the
TPM output result into it.

Prevents a kernel panic in case the user tries to read the
pubek from sysfs.

Signed-off-by: Rajiv Andrade <srajiv@linux.vnet.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>