[media] em28xx: fix usb alternate setting for analog and digital video endpoints > 0

The current code assumes that the analog + digital video endpoints are always at
interface number 0 when changing the alternate setting.
This seems to work fine for most existing devices.
However, at least the SpeedLink VAD Laplace webcam has the video endpoint on
interface number 3 (which fortunately doesn't cause any trouble because ist uses
bulk transfers only).
We already consider the actual interface number for audio endpoints, so
rename the the audio_ifnum variable and use it for all device types.
Also get get rid of a pointless (ifnum < 0) in em28xx-audio.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

drivers/media/usb/em28xx/em28xx-dvb.c

@@ -203,7 +203,7 @@ static int em28xx_start_streaming(struct em28xx_dvb *dvb)
 		dvb_alt = dev->dvb_alt_isoc;
 	}
 
-	usb_set_interface(dev->udev, 0, dvb_alt);
+	usb_set_interface(dev->udev, dev->ifnum, dvb_alt);
 	rc = em28xx_set_mode(dev, EM28XX_DIGITAL_MODE);
 	if (rc < 0)
 		return rc;