tpm: suppress durations sysfs output if not read

Suppress the output in the 'durations' sysfs entry if they were not read
during driver initialization. This is similar to other sysfs entries
that return nothing if for some reason sending the commands to the TPM
fails.

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>