Silence compiler warnings on unsigned/signed comparison

payload_size and throughput_pktsize are really only valid as unsigned
integers, so mark them as such.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
diff --git a/sigma_dut.c b/sigma_dut.c
index 56098e1..0ba5dc0 100644
--- a/sigma_dut.c
+++ b/sigma_dut.c
@@ -845,7 +845,7 @@
 			break;
 		case 'T':
 			sigma_dut.throughput_pktsize = atoi(optarg);
-			if (sigma_dut.throughput_pktsize <= 0) {
+			if (sigma_dut.throughput_pktsize == 0) {
 				printf("Invalid -T value\n");
 				exit(0);
 			}