dmi.c: make sure we call isprint() correctly

ISO C and POSIX require to call ctype functions with values representable
by unsigned char. We have used a char as input so far which might be
negative and hence get sign-extended.

Corresponding to flashrom svn r1852.

Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
1 file changed