isdn/mISDN: add __init/__exit macros to dsp_core.c

Trivial patch which adds the __init/__exit macros to the module_init/
module_exit functions of

drivers/isdn/mISDN/dsp_core.c

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>