tpm_tis: Return 0 for every register in case of failure mode

Rather than returning ~0, return 0 for every register in case of failure
mode. The '0' is better to indicate that there's no device there. It avoids
SeaBIOS detecting a device and getting stuck on it trying to read and write
its registers.

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

hw/tpm/tpm_tis.c

@@ -545,7 +545,7 @@ static uint64_t tpm_tis_mmio_read(void *opaque, hwaddr addr,
     uint8_t v;
 
     if (tpm_backend_had_startup_error(s->be_driver)) {
-        return val;
+        return 0;
     }
 
     switch (offset) {