hwmon: (adt7470) Create functions for updating readings and limits

Split the body of adt7470_update_device() into two helper functions
adt7470_update_sensors() and adt7470_update_limits(). Although neither
of the new helpers returns an error yet lay the groundwork for
propagating failures through to the sysfs readers.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Link: https://lore.kernel.org/r/20201019223423.31488-2-chris.packham@alliedtelesis.co.nz
Signed-off-by: Guenter Roeck <linux@roeck-us.net>