ftmod_misdn: Rework channel de-/activation and activate d-channel early.

ftmod_misdn: Rework channel de-/activation and activate d-channel early.

The former fixes a strange "bug" with hfcsusb, where a b-channel deactivation
on a inactive channel (caused by a reset cycle) would cause the port to
lock up and stop processing events.

NOTE: this still needs to be investigated further, but this workaround will
      at least prevent it from breaking completely.

We'll now keep track of the channel activation state and not send any
PH_ACTIVATE_REQ / PH_DEACTIVATE_REQ requests, if the channel already has the
desired state.

Signed-off-by: Stefan Knoblich <stkn@openisdn.net>