hwmon: (ads7828) driver cleanup

As there is no reliable way to identify the chip, it is preferable to
remove the detect callback, to avoid misdetection.

Module parameters are not worth it here, so let's get rid of them and
add an ads7828_platform_data structure instead.

Clean the code by removing unused macros, fixing coding style issues,
avoiding function prototypes and using convenient macros such as
module_i2c_driver().

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>