[coreboot] [libpayload] Fix drawing chars

[Jordan Crouse]

Somewhere in the layers of functions we're accidently casting unsigned
character values as signed, and they are getting sign extended, causing
blinking and bold text.  This patch makes sure we only use the 8 bits we
support right now.

This is probably not wchar safe, as the comment says.

-- 
Jordan Crouse
Systems Software Development Engineer 
Advanced Micro Devices, Inc.