media: em28xx: fix a regression with HVR-950

Commit be7fd3c3 ("media: em28xx: Hauppauge DualHD second tuner
functionality") removed the logic with sets the alternate for the DVB
device. Without setting the right alternate, the device won't be
able to submit URBs, and userspace fails with -EMSGSIZE:

	ERROR     DMX_SET_PES_FILTER failed (PID = 0x2000): 90 Message too long

Tested with Hauppauge HVR-950 model A1C0.

Fixes: be7fd3c3 ("media: em28xx: Hauppauge DualHD second tuner functionality")

Cc: Brad Love <brad@nextdimension.cc>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

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

@@ -199,6 +199,7 @@ static int em28xx_start_streaming(struct em28xx_dvb *dvb)
 	int rc;
 	struct em28xx_i2c_bus *i2c_bus = dvb->adapter.priv;
 	struct em28xx *dev = i2c_bus->dev;
+	struct usb_device *udev = interface_to_usbdev(dev->intf);
 	int dvb_max_packet_size, packet_multiplier, dvb_alt;
 
 	if (dev->dvb_xfer_bulk) {
@@ -217,6 +218,7 @@ static int em28xx_start_streaming(struct em28xx_dvb *dvb)
 		dvb_alt = dev->dvb_alt_isoc;
 	}
 
+	usb_set_interface(udev, dev->ifnum, dvb_alt);
 	rc = em28xx_set_mode(dev, EM28XX_DIGITAL_MODE);
 	if (rc < 0)
 		return rc;