isdn: mISDN: remove unneeded variable 'ret'

Fix the following coccicheck warning:
./drivers/isdn/mISDN/dsp_core.c:956:6-9: Unneeded variable: "err".
Return "0" on line 1001

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>