hwmon: (tmp401) Hide register write address differences in regmap code

Since we are using regmap access functions to write into chip registers,
we can hide the difference in register write addresses within regmap
code. By doing this we do not need to clear the regmap cache on register
writes, and the high level code does not need to bother about different
read/write register addresses.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>