hwmon: (ads7828) add support for ADS7830

The ADS7830 device is almost the same as the ADS7828,
except that it does 8-bit sampling, instead of 12-bit.
This patch extends the ads7828 driver to support this chip.

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