Check for INTERFACE or SMAC in dtp setup

The module requires one of these to be set, otherwise you get failures like:

```
[-] Auxiliary failed: TypeError wrong argument type nil (expected String)
[-] Call stack:
[-]   /home/jhart/labs/git/metasploit-framework/lib/msf/core/exploit/capture.rb:478:in `addresses'
[-]   /home/jhart/labs/git/metasploit-framework/lib/msf/core/exploit/capture.rb:478:in `get_mac'
[-]   /home/jhart/labs/git/metasploit-framework/modules/auxiliary/spoof/cisco/dtp.rb:59:in `smac'
[-]   /home/jhart/labs/git/metasploit-framework/modules/auxiliary/spoof/cisco/dtp.rb:64:in `run'
```

## Validation

 - [ ] Use module before this PR, confirming that it fails when neither ```INTERFACE``` or ```SMAC``` are set
 - [ ] Use module before this PR, confirming that it works properly when either ```INTERFACE``` or ```SMAC``` are set
 - [ ] Use this module after this PR, confirming that the failure is more sane when ```INTERFACE``` or ```SMAC``` are not set