ASoC: SOF: Fix memory leak in sof_dfsentry_write

In the implementation of sof_dfsentry_write() memory allocated for
string is leaked in case of an error. Go to error handling path if the
d_name.name is not valid.

Fixes: 091c12e1f50c ("ASoC: SOF: debug: add new debugfs entries for IPC flood test")
Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Link: https://lore.kernel.org/r/20191027194856.4056-1-navid.emamdoost@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>
