tpm: Fix expected number of response bytes of TPM1.2 PCR Extend

The TPM1.2 PCR Extend operation only returns 20 bytes in the body,
which is the size of the PCR state.

This fixes a problem where IMA gets errors with every PCR Extend.

Fixes: c659af78 ("tpm: Check size of response before accessing data")
Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Acked-by: Mimi Zohar <zohar@us.ibm.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>