hwmon: (lm90) Don't spam the kernel log

Degrade the "Unsupported chip" message from info to debug level.
There's nothing wrong with this, so no need to bother the user.
Also make the message slightly more descriptive.

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