staging:iio: Use userspace types for iio_event_data

Since we want to export struct iio_event_data to userspace use the userspace
integer types. Also add a include to linux/types.h.

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