hwmon: (ads1015) Fix off-by-one for valid channel index checking

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>
Cc: stable@vger.kernel.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>