libfdt: fdt_get_string(): Fix sequential write comparison warnings

With -Wsign-compare, compilers warn about a mismatching signedness in
comparisons in fdt_get_string().

Introduce a new usigned variable, which holds the actual (negated)
stroffset value, so we avoid negating all the other variables and have
proper types everywhere.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20201001164630.4980-6-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

libfdt/fdt_ro.c

@@ -67,11 +67,13 @@ const char *fdt_get_string(const void *fdt, int stroffset, int *lenp)
 				len = fdt_size_dt_strings(fdt) - stroffset;
 		}
 	} else if (fdt_magic(fdt) == FDT_SW_MAGIC) {
-		if ((stroffset >= 0)
-		    || (stroffset < -fdt_size_dt_strings(fdt)))
+		unsigned int sw_stroffset = -stroffset;
+
+		if ((stroffset >= 0) ||
+		    (sw_stroffset > fdt_size_dt_strings(fdt)))
 			goto fail;
-		if ((-stroffset) < len)
-			len = -stroffset;
+		if (sw_stroffset < len)
+			len = sw_stroffset;
 	} else {
 		err = -FDT_ERR_INTERNAL;
 		goto fail;