Fix sp_openserport users on Windows

Corresponding to flashrom svn r1627.

Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Idwer Vollering <vidwer@gmail.com>
diff --git a/pony_spi.c b/pony_spi.c
index 7875200..101751f 100644
--- a/pony_spi.c
+++ b/pony_spi.c
@@ -116,7 +116,7 @@
 	arg = extract_programmer_param("dev");
 	if (arg && strlen(arg)) {
 		sp_fd = sp_openserport(arg, 9600);
-		if (sp_fd < 0) {
+		if (sp_fd == SER_INV_FD) {
 			free(arg);
 			return 1;
 		}