accept DAHDI channels as valid channels

reason: when answering a DAHDI call from switchboard, Asterisk sends
two consecutive events:

- Bridge Unlink between the switchboard phone and the DAHDI channel
- Bridge Link between the same channels

Considering DAHDI channels as invalid blocked the processing of the
Bridge Link event, making the DAHDI call untracked by the CTI. This
blocked the action of hanging up the call from the switchboard.