drivers/spi/tpm: honor tis_sendrecv() API

The spi tis_sendrecv() implementation was always returning success
for all transactions. Correct this by returning -1 on error when
tpm2_process_command() returns 0 since that's its current failure
return code.

BUG=b:36598499

Change-Id: I8bfb5a09198ae4c293330e770271773a185d5061
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/19058
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins)

src/drivers/spi/tpm/tis.c

@@ -85,6 +85,11 @@ int tis_sendrecv(const uint8_t *sendbuf, size_t sbuf_size,
 		 uint8_t *recvbuf, size_t *rbuf_len)
 {
 	int len = tpm2_process_command(sendbuf, sbuf_size, recvbuf, *rbuf_len);
+
+	if (len == 0)
+		return -1;
+
 	*rbuf_len = len;
+
 	return 0;
 }