hwmon: (adt7475) Use enum chips when loading attenuator settings

Make use of enum chips and use a switch statement in load_attenuators()
so that the compiler can tell us if we've failed to cater for a
supported chip.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20220323034056.260455-4-chris.packham@alliedtelesis.co.nz


Signed-off-by: Guenter Roeck <linux@roeck-us.net>