hwmon: (lm90) Don't access nonexistent registers on Maxim chips

The Maxim chips supported by the lm90 driver have 8-bit high and low
remote limit values, not 11-bit as the other chips have. So stop reading
from and writing to registers that do not exist on these chips. Also
round the limit values set by the user properly.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Martyn Welch <martyn.welch@gefanuc.com>