tpm: add buffer function to point to returned parameters

Replace all instances of &buf.data[TPM_HEADER_SIZE] with a new
function tpm_buf_parameters() because encryption sessions change
where the return parameters are located in the buffer since if a
return session is present they're 4 bytes beyond the header with those
4 bytes giving the parameter length.  If there is no return session,
then they're in the usual place immediately after the header.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>