Clearing spanning tree counters

Clear spanning tree counters on all interfaces.
```
device# clear spanning-tree counter
```
Clear spanning tree counters on a specified Ethernet interface.
```
device# clear spanning-tree counter interface ethernet 0/3
```
Clear spanning tree counters on a specified port channel interface.
```
device# clear spanning-tree counter interface port-channel 12
```
Port channel interface numbers range from 1 through 64.