mISDN: use kstrdup() in dsp_pipeline_build

Use kstrdup instead of strlen-kmalloc-strcpy. Remove unneeded NULL
test, it will be tested inside kstrdup. Remove 0 length string test,
it has been tested in the caller of dsp_pipeline_build.

Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: David S. Miller <davem@davemloft.net>