tpm: add missing parameter in private data structure description

Both parameters 'duration_ms' and 'retry_time_ms' of the tpm_chip_priv
structure are documented is the comment above the declaration but 'buf'
was forgotten. Add the missing description.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>