hwmon: (adc128d818) Drop write support on inX_input attributes

Writes into input registers doesn't make sense, even more so since
the writes actually ended up writing into the maximum limit registers.
Drop it.

Cc: stable@vger.kernel.org
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>