iio: light: rpr0521 channel numbers reordered

Move proximity channel from last to first in structs to avoid confusion
later with buffered triggers. Proximity data output is first in rpr0521
register map.

Signed-off-by: Mikko Koivunen <mikko.koivunen@fi.rohmeurope.com>
Acked-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>