camellia: fix camellia_self_test failure on compilers where char is unsigned by default
Fix camellia_self_test failure when compiled on compilers
where char is unsigned by default (on gcc this mode is
enabled with -funsigned-char).
This also fixes these warnings:
camellia.c:433: warning: comparison is always true due
to limited range of data type
camellia.c:437: warning: comparison is always true due
to limited range of data type
camellia.c:441: warning: comparison is always true due
to limited range of data type
camellia.c:445: warning: comparison is always true due
to limited range of data type
camellia.c:450: warning: comparison is always true due
to limited range of data type