coresight-etm: unlock on error paths in mode_store()

There are some missing unlocks on the error paths.

Fixes: a939fc5a ('coresight-etm: add CoreSight ETM/PTM driver')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>