Fix compilation with MinGW

This was broken since r1557 when we got rid of some exit calls, but returned
-1 instead which is not a valid HANDLE value.

Corresponding to flashrom svn r1591.

Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Idwer Vollering <vidwer@gmail.com>

programmer.h

@@ -636,8 +636,10 @@ void serprog_delay(int usecs);
 /* serial.c */
 #ifdef _WIN32
 typedef HANDLE fdtype;
+#define SER_INV_FD	INVALID_HANDLE_VALUE
 #else
 typedef int fdtype;
+#define SER_INV_FD	-1
 #endif
 
 void sp_flush_incoming(void);