iio: hid-sensors-prox: Make proximity channel indexed

We are going to introduce more proximity channels. Make proximity a
indexed channel now, in a simple patch, so the change can be easily
bisected if there are any issues.

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Link: https://patch.msgid.link/20241101-hpd-v3-4-e9c80b7c7164@chromium.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>