hwmon: (adt7475) Change show functions to return error data correctly

Change update device function to return an error pointer if needed,
and report the error to user space.

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: Clarified/updated description]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>