libfdt: fdt_add_string_(): Fix comparison warning

With -Wsign-compare, compilers warn about a mismatching signedness
in a comparison in fdt_add_string_().

Make all variables unsigned, and express the negative offset trick via
subtractions in the code.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20201001164630.4980-2-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

libfdt/fdt_sw.c

@@ -250,18 +250,18 @@ int fdt_end_node(void *fdt)
 static int fdt_add_string_(void *fdt, const char *s)
 {
 	char *strtab = (char *)fdt + fdt_totalsize(fdt);
-	int strtabsize = fdt_size_dt_strings(fdt);
+	unsigned int strtabsize = fdt_size_dt_strings(fdt);
-	int len = strlen(s) + 1;
+	unsigned int len = strlen(s) + 1;
-	int struct_top, offset;
+	unsigned int struct_top, offset;
 
-	offset = -strtabsize - len;
+	offset = strtabsize + len;
 	struct_top = fdt_off_dt_struct(fdt) + fdt_size_dt_struct(fdt);
-	if (fdt_totalsize(fdt) + offset < struct_top)
+	if (fdt_totalsize(fdt) - offset < struct_top)
 		return 0; /* no more room :( */
 
-	memcpy(strtab + offset, s, len);
+	memcpy(strtab - offset, s, len);
 	fdt_set_size_dt_strings(fdt, strtabsize + len);
-	return offset;
+	return -offset;
 }
 
 /* Must only be used to roll back in case of error */