staging:iio: Add ability to allocate private data space to iio_allocate_device

Suggested by Arnd Bergmann.  Note this will break ALL drivers that
are out of mainline.  The fix is trivial change of
iio_allocate_device() -> iio_allocate_device(0)
Sorry if this causes issues for any one!

V2: Include new drivers in the update

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