hwmon: (lm90) Support MAX6646, MAX6647 and MAX6649

These Maxim chips are similar to MAX6657 but use unsigned temperature
values to allow for readings up to 145 degrees.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>