mISDN: comment out unused symbols

Fix this sparse warnings:

  drivers/isdn/hardware/mISDN/hfcmulti.c:794: warning: 'vpm_check' defined but not used
  drivers/isdn/mISDN/dsp_cmx.c:1546: warning: 'dsp_start_jiffies' defined but not used

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