TPM: fix transmit_cmd error logic

It's incorrect to assume that buffers returned by the TPM
10 bytes long are always error reports. This patches
parses the error field in its header instead. The error report
is now being printed using dev_err() instead of dev_dbg(), making
it easier for users to provide more detailed bug reports.

Signed-off-by: Rajiv Andrade <srajiv@linux.vnet.ibm.com>

drivers/char/tpm/tpm.c

@@ -438,7 +438,6 @@ out:
 }
 
 #define TPM_DIGEST_SIZE 20
-#define TPM_ERROR_SIZE 10
 #define TPM_RET_CODE_IDX 6
 
 enum tpm_capabilities {
@@ -467,12 +466,14 @@ static ssize_t transmit_cmd(struct tpm_chip *chip, struct tpm_cmd_t *cmd,
 	len = tpm_transmit(chip,(u8 *) cmd, len);
 	if (len <  0)
 		return len;
-	if (len == TPM_ERROR_SIZE) {
-		err = be32_to_cpu(cmd->header.out.return_code);
-		dev_dbg(chip->dev, "A TPM error (%d) occurred %s\n", err, desc);
-		return err;
-	}
-	return 0;
+	else if (len < TPM_HEADER_SIZE)
+		return -EFAULT;
+
+	err = be32_to_cpu(cmd->header.out.return_code);
+	if (err != 0)
+		dev_err(chip->dev, "A TPM error (%d) occurred %s\n", err, desc);
+
+	return err;
 }
 
 #define TPM_INTERNAL_RESULT_SIZE 200