staging:iio:events: Use waitqueue lock to protect event queue

staging:iio:events: Use waitqueue lock to protect event queue

Use the waitqueue lock to protect the event queue instead of a custom mutex.
This has the advantage that we can call the waitqueue operations with the lock
held, which simplifies the code flow a bit.

Signed-off-by: Lars-Peter Clausen <[email protected]>
Acked-by: Jonathan Cameron <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>