tpm: Open code tpm_buf_parameters()

With only single call site, this makes no sense (slipped out of the
radar during the review). Open code and document the action directly
to the site, to make it more readable.

Fixes: 1b6d7f9e

 ("tpm: add session encryption protection to tpm2_get_random()")
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>