niimaqdx: allow Mono16 to be big or little endian depending on interface

It appears FireWire and GigE Vision use different endianness. So now we need
to keep track of two things, PixelFormat and endianness.

sys/niimaqdx/gstniimaqdx.h

@@ -49,6 +49,7 @@ typedef struct _GstNiImaqDxSrcClass GstNiImaqDxSrcClass;
 typedef struct
 {
     const char *pixel_format;
+    int endianness;
     const char *gst_caps_string;
     int bpp;
     int depth;