tpm: tpm2_probe: propagate transport errors

tpm2_probe() reads the protocol version from the response tag and is
documented to return -errno on failure, but it always returns 0.  A
transport error (tpm_transmit_cmd() < 0) is thus swallowed together with
the intentionally ignored TPM response code, leaving TPM_CHIP_FLAG_TPM2
unset. A transient bus failure on the first command then misdetects a
TPM 2.0 as a 1.2 and the probe fails on the TPM 1.2 path:

  tpm_tis_i2c 0-002e: 1.2 TPM (device-id 0x1C, rev-id 22)
  tpm tpm0: A TPM error (-62) occurred attempting to determine the timeouts

Return the transport error instead; rc >= 0 still means a valid header was
received and the TPM's own response code is still ignored.

Fixes: 94e266ba1f ("tpm: migrate tpm2_probe() to use struct tpm_buf")
Cc: stable@vger.kernel.org
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/20261001060114.1582092-1-alexander.sverdlin@siemens.com
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

drivers/char/tpm/tpm2-cmd.c

@@ -483,8 +483,9 @@ int tpm2_probe(struct tpm_chip *chip)
 		out = (struct tpm_header *)buf->data;
 		if (be16_to_cpu(out->tag) == TPM2_ST_NO_SESSIONS)
 			chip->flags |= TPM_CHIP_FLAG_TPM2;
+		return 0;
 	}
-	return 0;
+	return rc;
 }
 EXPORT_SYMBOL_GPL(tpm2_probe);