hwmon: (it87) Split out chip registers setting code on probe path

This commit splits out chip registers setting code on probe path to
separate functions so they can be reused for setting the device properly
again when system resumes from suspend.

While we are at it let's also make clear that on IT8720 and IT8782 it's
the VCCH5V line that is (possibly) routed to in7.
This will make it consistent with a similar message that it printed on
IT8783.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>