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>

serprog.c

@@ -372,7 +372,7 @@ int serprog_init(void)
 		}
 		if (strlen(device)) {
 			sp_fd = sp_openserport(device, atoi(baudport));
-			if (sp_fd < 0) {
+			if (sp_fd == SER_INV_FD) {
 				free(device);
 				return 1;
 			}