tpm: Check the bios_dir entry for NULL before accessing it

Check the bios_dir entry for NULL before accessing it. Currently
this crashes the driver when a TPM 2 is attached and the entries
are NULL.

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Tested-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>