m68k: use correct variable name in boot info string macro

Every time this macro is used, the caller is passing in
"parameters_base", so this bug wasn't spotted. But the actual macro
variable name is "base", so use that instead.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20220625152318.120849-1-Jason@zx2c4.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

hw/m68k/bootinfo.h

@@ -54,6 +54,6 @@
             stb_phys(as, base++, string[i]); \
         } \
         stb_phys(as, base++, 0); \
-        base = (parameters_base + 1) & ~1; \
+        base = (base + 1) & ~1; \
     } while (0)
 #endif