dtc: Fix signedness comparisons warnings: Wrap (-1)

With -Wsign-compare, compilers warn about a mismatching signedness
in a comparison in dtc's data_copy_file().

Even though maxlen is of an unsigned type, we compare against "-1",
which is passed in from the parser to indicate an unknown size.

Cast the "-1" to an unsigned size to make the comparison match.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20201012161948.23994-9-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

data.c

@@ -84,7 +84,7 @@ struct data data_copy_file(FILE *f, size_t maxlen)
 	while (!feof(f) && (d.len < maxlen)) {
 		size_t chunksize, ret;
 
-		if (maxlen == -1)
+		if (maxlen == (size_t)-1)
 			chunksize = 4096;
 		else
 			chunksize = maxlen - d.len;