mISDN: Add missing release functions

Add missing release function of ec-devices. Each device require a relase
function now. All destruction (memory and list entry) must be done within
the given release function of device, rather than after unregistering device.

Signed-off-by: Andreas Eversberg <andreas@eversberg.eu>
Signed-off-by: Karsten Keil <kkeil@suse.de>