commit 56de1377 upstream.

Current code uses channel as array index, so the valid channel value is
0 .. ADS1015_CHANNELS - 1.
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>