ASoC: SOF: debug: Fix a potential issue on string buffer termination

ASoC: SOF: debug: Fix a potential issue on string buffer termination

The function simple_write_to_buffer() doesn't add string termination
at the end of buf, we need to handle it on our own. This change refers
to the function tokenize_input() in debug.c and the function
sof_dfsentry_trace_filter_write() in trace.c.

Fixes: 091c12e1f50c ("ASoC: SOF: debug: add new debugfs entries for IPC flood test")
Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Signed-off-by: Hui Wang <hui.wang@canonical.com>
Link: https://lore.kernel.org/r/20210208103857.75705-1-hui.wang@canonical.com
Signed-off-by: Mark Brown <broonie@kernel.org>