mISDN: Fix NULL pointer bug in if-condition of mISDN_dsp

Fix a bug (was introduced by a cut & paste error)
in cases when dsp->conf was NULL.

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