staging:iio:adc:ad7298 move to irqchip based trigger handling.

Untested. This one is of a different form, so worth a closer look than
the previous incredibly similar patches (which were based on the
adis16400 that I have tested).

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