hwmon: (tmp102) Convert to use regmap, and drop local cache

By converting the driver to regmap, we can use regmap to cache non-volatile
registers. Stop caching the temperature register; while potentially reading
it more often can result in reading it more often than necessary, this is
offset by the gain due to not re-reading the limit registers.

A positive side effect of this change is that limit registers can now be
read and updated before the first temperature conversion is complete.

Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>