commit e9814295 upstream.

Current code uses data_rate as array index in ads1015_read_adc() and uses pga
as array index in ads1015_reg_to_mv, so we must make sure both data_rate and
pga settings are in valid value range.
Return -EINVAL if the setting is out-of-range.
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>