Give V4L2_PIX_FMT_MJPEG an unique name

This patch gives `V4L2_PIX_FMT_MJPEG` an unique name. Previously, `V4L2_PIX_FMT_MJPEG` would be chosen instead of `V4L2_PIX_FMT_JPEG` if `JPEG` was specified on the command line.

Relates to issue #40

src/ustreamer/device.c

@@ -41,7 +41,7 @@ static const struct {
 	{"UYVY",	V4L2_PIX_FMT_UYVY},
 	{"RGB565",	V4L2_PIX_FMT_RGB565},
 	{"RGB24",	V4L2_PIX_FMT_RGB24},
-	{"JPEG",	V4L2_PIX_FMT_MJPEG},
+	{"MJPEG",	V4L2_PIX_FMT_MJPEG},
 	{"JPEG",	V4L2_PIX_FMT_JPEG},
 };