staging:iio: buffer example - add lots more runtime parameters

Add ability to control delay for event free buffers
Add ability to control length of buffer
Add ability to control how many read cycles occur

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Tested-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>