mISDN: use NULL pointer instead of plain integer

Fix more than 100 (all remaining in mISDN) sparse warnings:

  drivers/isdn/hardware/mISDN/hfcmulti.c:***:**: warning: Using plain integer as NULL pointer
  drivers/isdn/mISDN/dsp_tones.c:***:**: warning: Using plain integer as NULL pointer
  drivers/isdn/mISDN/dsp_pipeline.c:***:**: warning: Using plain integer as NULL pointer

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Acked-by: Karsten Keil <kkeil@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>