processor: Change `Step` default value to USHRT_MAX

Change `Step` default value to USHRT_MAX due to
https://gerrit.openbmc.org/c/openbmc/phosphor-dbus-interfaces/+/60717

For a new processor, it's possible that the stepping is really `0`. In
this case, the `Step` will still be ignored which is not expected.

https://gerrit.openbmc.org/c/openbmc/phosphor-dbus-interfaces/+/60717


changes the `Step` default value to `maxint`. Thus the `Step` can
correctly display 0 on redfish.

Tested:
1. Redfish service validator passing.
2. bmcweb works fine without errors.
3. Step was hidden when its value is 65535 (max uint16_t).
4. Step also displayed even the value is `0`.
```
root@machine:~# curl localhost/redfish/v1/Systems/system/Processors/cpu0
    ...
    "Step": "0x0000"
    ...
```

Change-Id: I1857fc597ef2ab13256dc60e534c23d452e5d695
Signed-off-by: Michael Shen <gpgpgp@google.com>