disas/m68k: clean up local variable shadowing

Fix following warnings

.../disas/m68k.c: In function ‘print_insn_arg’:
.../disas/m68k.c:1635:13: warning: declaration of ‘val’ shadows a previous local [-Wshadow=compatible-local]
 1635 |         int val = fetch_arg (buffer, place, 5, info);
      |             ^~~
.../disas/m68k.c:1093:7: note: shadowed declaration is here
 1093 |   int val = 0;
      |       ^~~

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Message-ID: <20230925084455.395150-1-laurent@vivier.eu>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>