libfdt: fdt_grab_space_(): Fix comparison warning

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

All the involved values cannot be negative, so let's switch the types of
the local variables to unsigned to make the compiler happy.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20200921165303.9115-4-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

libfdt/fdt_sw.c

@@ -93,8 +93,8 @@ static inline uint32_t sw_flags(void *fdt)
 
 static void *fdt_grab_space_(void *fdt, size_t len)
 {
-	int offset = fdt_size_dt_struct(fdt);
-	int spaceleft;
+	unsigned int offset = fdt_size_dt_struct(fdt);
+	unsigned int spaceleft;
 
 	spaceleft = fdt_totalsize(fdt) - fdt_off_dt_struct(fdt)
 		- fdt_size_dt_strings(fdt);