staging: ozwpan: Include oz_events_clear() conditionally

oz_events_clear() is referenced only when CONFIG_DEBUG_FS is defined.
Move the definition too under this flag.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Rupesh Gujare <rgujare@ozmodevices.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>