ipmid: Make sure NcsiCable doesn't reset ipmi tree

The NcsiCable dbus object is not a sensor and doesn't export any
sensors. This can potentially reset the ipmi tree when the ncsi
interface gets restarted in the middle of running machines.

Tested:
Manually set the NCSI link up/down
```
$ ip link set ncsi down
$ ip link set ncsi up
```

Entity Manager removed/added dbus object
```
Sep 30 20:50:35 gbmc  entity-manager[4361]: Inventory Removed: NcsiCable
Sep 30 20:52:06 gbmc entity-manager[4361]: Inventory Added: NcsiCable
```

Ipmid didn't log that ipmi cache tree got reset.
```
Sep 30 20:50:41 gbmc ipmid[30820]: fail to update subtree
Sep 30 20:50:41 gbmc ipmid[30820]: fail to update subtree
```

Fusion-Link:
- platform11: http://fusion2/924dbcb8-44df-3ea0-8d88-d65b21b50c62
Google-Bug-Id: 448205609
Platforms-Affected: All
Change-Id: I75025c1cb65cf6be03e1ab0c8838d6d2e7301535
Signed-off-by: Willy Tu <wltu@google.com>
(cherry picked from commit 4b098eee322dd3c62597687f6d884072c39d946a)