mkhtemp: fix bad check

where the path is quite short and the number of X
is quite big compared to the rest of it, this
check will actually cause a false overflow
error. the maths are correct, just not the error

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

util/nvmutil/lib/file.c

@@ -842,8 +842,7 @@ mkhtemp(int *fd,
 		return -1;
 	}
 
-	if (fname_len > len ||
-	    fname_len > (len - xc)) {
+	if (fname_len > len) {
 		errno = EOVERFLOW;
 		return -1;
 	}