hwmon: (tmp401) Simplification and cleanup

Use two-dimensional array pointing to registers
Merge temperature and limit access functions into a single function
Return error codes from I2C reads
Use DIV_ROUND_CLOSEST for rounding operations and improve rounding

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Jean Delvare <khali@linux-fr.org>