tpm2: remove unused buffer size definition

TPM2 structure definitions use pointers instead of buffers where
possible. One structure was left behind. Replace that buffer definition
with a pointer to be consistent.

BRANCH=none
BUG=chrome-os-partner:50645
TEST=compilation succeeds, the code using the changed structure in the
     upcoming patches allows to successfully boot chrome OS on Kevin

Change-Id: Iea59943aa0ad6e42fcd479765a9ded0d7a1680d7
Signed-off-by: Martin Roth <martinroth@chromium.org>
Original-Commit-Id: 02b2909b18
Original-Change-Id: I9856ac516be13f5892ba8af0526708409a297033
Original-Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/358771
Original-Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-on: https://review.coreboot.org/15609
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/lib/tpm2_tlcl_structures.h

@@ -198,7 +198,7 @@ typedef struct {
 typedef union {
 	struct {
 		uint16_t  size;
-		uint8_t   buffer[sizeof(TPMU_HA)];
+		const uint8_t   *buffer;
 	} t;
 	TPM2B b;
 } TPM2B_DIGEST;