Leica LIF: do not set the 'rgb' flag for 16-bit multi-channel data

Multi-channel images with more than 8 bits per pixel are not stored as
RGB images, even though the stored byte count suggests that they might
be.

See QA 7695 and
http://lists.openmicroscopy.org.uk/pipermail/ome-users/2013-November/004051.html

To test, verify that the images in QA 7695 look correct and that the builds are green.