util/nvmutil: better SSIZE_MAX define

the old one assumes that ssize_t is signed size_t,
which let's face it, is always true in practise,
but not actually guaranteed!

so now i'm using one that's even more pedantic.

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/nvmutil/nvmutil.c

@@ -285,12 +285,8 @@ static const char *mac_str;
 static const char *fname;
 static const char *argv0;
 
-#ifndef SIZE_MAX
-#define SIZE_MAX ((size_t)-1)
-#endif
-
 #ifndef SSIZE_MAX
-#define SSIZE_MAX ((ssize_t)(SIZE_MAX >> 1))
+#define SSIZE_MAX ((ssize_t)(~((size_t)1 << (sizeof(ssize_t)*CHAR_BIT-1))))
 #endif
 
 /*