hwmon: (adt7475) Split device update function to measure and limits

The update function reads both measurement and limit values.
Those parts can be split so split them for a maintainability.

Signed-off-by: Tokunori Ikegami <ikegami@allied-telesis.co.jp>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Chris Packham <chris.packham@alliedtelesis.co.nz>
[groeck: Clarify description]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>