Fix a bug that always caused ubus call release from dectmngr replying error

Each time when dectmngr releases a call via ubus, an error reply is received like below.
{
	"terminal": 1,
	"pcm": 0,
	"errno": 1
}
Command failed: Unknown error

This doesn't cause any functional loss since dectmngr doesn't check the ubus reply from
voicemngr. But it is still misleading, especially when the ubus call is invoked manually
for debug and test purpose.