video/console: Fix DM_VIDEO font glyph array indexing

When the character to be printed on a DM_VIDEO console is from the
"extended ASCII" range (0x80 - 0xff), it will be treated as a negative
number, as it's declared as a signed char. This leads to negative array
indicies into the glyph bitmap array, and random garbled characters.

Cast the character to an unsigned type to make the index always positive
and avoid an out-of-bounds access.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>