mISDN: fix sparse warning: symbol 'nskb' shadows an earlier one

Impact: define first occurence of variable 'nskb' in inner most possible scope

Fix this sparse warning:

  drivers/isdn/mISDN/dsp_core.c:746:20: warning: symbol 'nskb' shadows an earlier one
  drivers/isdn/mISDN/dsp_core.c:634:19: originally declared here

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