tpm: use tpm_pcr_read_dev() in tpm_do_selftest()

Instead of a ad-hoc protocol message construction it is better to
call tpm_pcr_read_dev().

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Reviewed-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>