Remove pri_destroycall() in on_hangup() and
use pri_hangup() instead of the deprecated pri_release().

NOTE: The original patch used PRI_CAUSE_NORMAL_CIRCUIT_CONGESTION for the cause
      code, which (without pri_enable_hangup_fix() enabled) emits a RELEASE COMPLETE
      message. This version of the patch uses PRI_CAUSE_NORMAL_CLEARING (-1). (stkn)

Additionally, use ftdm_set_state() instead of ftdm_set_state_locked() because the channel
is already locked. (stkn)
Completely-untested-by: Stefan Knoblich <stkn@openisdn.net>