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>

include/tpm-common.h

@@ -35,6 +35,7 @@ enum tpm_duration {
  *
  * @duration_ms:	Length of each duration type in milliseconds
  * @retry_time_ms:	Time to wait before retrying receive
+ * @buf:		Buffer used during the exchanges with the chip
  */
 struct tpm_chip_priv {
 	uint duration_ms[TPM_DURATION_COUNT];