ASoC: SOF: debug: fix possible memory leak in sof_dfsentry_write()

ASoC: SOF: debug: fix possible memory leak in sof_dfsentry_write()

'string' is malloced in sof_dfsentry_write() and should be freed
before leaving from the error handling cases, otherwise it will cause
memory leak.

Fixes: 6b8202764934 ("ASoC: SOF: debug: add new debugfs entries for IPC flood test")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Link: https://lore.kernel.org/r/20190705081637.157169-1-weiyongjun1@huawei.com
Signed-off-by: Mark Brown <broonie@kernel.org>