staging:iio: treewide rename iio_triggered_ring_* to iio_triggered_buffer_*

Not always a ring so naming is missleading.
Also, kfifo_buf is probably first buffer to take out of staging and it
definitely isn't a ring.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>