[AX.25]: Fix signed char bug

On architectures where the char type defaults to unsigned some of the
arithmetic in the AX.25 stack to fail, resulting in some packets being dropped
on receive.

Credits for tracking this down and the original patch to
Bob Brose N0QBJ <linuxhams@n0qbj-11.ampr.org>.

Signed-off-by: Ralf Baechle DL5RB <ralf@linux-mips.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>