mISDN: Fix memory leak in dsp_hwec_enable()

mISDN: Fix memory leak in dsp_hwec_enable()

dsp_hwec_enable() allocates dup pointer by kstrdup(arg),
but then it updates dup variable by strsep(&dup, ",").
As a result when it calls kfree(dup), the dup variable may be
a modified pointer that no longer points to the original allocated
memory, causing a memory leak.

The issue is the same pattern as fixed in commit c6a502c22999
("mISDN: Fix memory leak in dsp_pipeline_build()").

Fixes: 9a4381618262 ("mISDN: Remove VLAs")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250828081457.36061-1-linmq006@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>