staging:iio:light: V3 fix out of bounds reg_cache[] access

V3 is a straightforward forward port to teh current tree of V2.

Simple fix is to just not cache REG_TEST (offset 8).
Cache doesn't help REG_TEST anyway since we write all 8 bits exactly once
(at resume/init time).

Also fix an "off-by-one" allocation of reg_cache[] array size that
was in the original code before I touched it.

Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Grant Grundler <grundler@chromium.org>
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>