hwmon: (tmp421) fix rounding for negative values

Old code produces -24999 for 0b1110011100000000 input in standard format due to
always rounding up rather than "away from zero".

Use the common macro for division, unify and simplify the conversion code along
the way.

Fixes: 9410700b ("hwmon: Add driver for Texas Instruments TMP421/422/423 sensor chips")
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Link: https://lore.kernel.org/r/20210924093011.26083-3-fercerpav@gmail.com


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