Input: ads7846 - convert to hwmon_device_register_with_groups()

Simplify the code and create mandatory 'name' attribute by using
new hwmon API.

Also use is_visible to determine visible attributes instead of creating
several different attribute groups.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>