Fix negative value for depths in Information Box

The get_depth_units function was expecting an unsigned int as a first parameter.
When it received a negative integer, the function made a cast to an unsigned int,
resulting in a very big number.

Signed-off-by: Nicu Badescu <badescunicu@yahoo.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>