mISDN: Fix DTMF locking bug issue

DTMF digits were sent up to socket in locked state.
Receive audio stream was not enabled in certain condition.

Signed-off-by: Andreas Eversberg <andreas@eversberg.eu>
Signed-off-by: Karsten Keil <keil@b1-systems.de>
Signed-off-by: David S. Miller <davem@davemloft.net>