staging:iio:adc:ad7606 conversion to irq_chip based polling.

I'm far from sure what the best way to handle this particular
part is, so have (I think) done the absolute minimum to change
it to the new interface.

V2: Trivial constification of device name.

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