hwmon: (lm90) Cache configuration register value

The configuration register does not change on its own. Yet, it is read
in various locations, modified, and written back. Simplify and optimize
the code by caching its value and by only writing it back when needed.

Cc: Boyang Yu <byu@arista.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>