staging:iio: push "sysfs.h" and linux/irq.h out of iio.h

These are no longer needed.
Requires a few driver updates for places "sysfs.h" should have been
present but wasn't.

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