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 <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>