[media] em28xx: fix analog streaming with USB bulk transfers

With the conversion to videobuf2, some unnecessary calls of
em28xx_set_alternate() have been removed. It is now called at analog streaming
start only.
This has unveiled a bug that causes USB bulk transfers to fail with all urbs
having status -EVOERFLOW.
The reason is, that for bulk transfers usb_set_interface() needs to be called
even if the previous alt setting was the same (side note: bulk transfers seem
to work only with alt=0).
While it seems to be NOT necessary for isoc transfers, it's reasonable to just
call usb_set_interface() unconditionally in em28xx_set_alternate().
Also add a comment that explains the issue to prevent regressions in the future.

Cc: stable@vger.kernel.org # for 3.8
Signed-off-by: NFrank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>

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

@@ -813,12 +813,12 @@ int em28xx_resolution_set(struct em28xx *dev)
 /* Set USB alternate setting for analog video */
 int em28xx_set_alternate(struct em28xx *dev)
 {
-	int errCode, prev_alt = dev->alt;
+	int errCode;
 	int i;
 	unsigned int min_pkt_size = dev->width * 2 + 4;
 
 	/* NOTE: for isoc transfers, only alt settings > 0 are allowed
-		 for bulk transfers, use alt=0 as default value */
+		 bulk transfers seem to work only with alt=0 ! */
 	dev->alt = 0;
 	if ((alt > 0) && (alt < dev->num_alt)) {
 		em28xx_coredbg("alternate forced to %d\n", dev->alt);
@@ -849,7 +849,9 @@ int em28xx_set_alternate(struct em28xx *dev)
 	}
 
 set_alt:
-	if (dev->alt != prev_alt) {
+	/* NOTE: for bulk transfers, we need to call usb_set_interface()
+	 * even if the previous settings were the same. Otherwise streaming
+	 * fails with all urbs having status = -EOVERFLOW ! */
 	if (dev->analog_xfer_bulk) {
 		dev->max_pkt_size = 512; /* USB 2.0 spec */
 		dev->packet_multiplier = EM28XX_BULK_PACKET_MULTIPLIER;
@@ -868,7 +870,6 @@ int em28xx_set_alternate(struct em28xx *dev)
 			      dev->alt, errCode);
 		return errCode;
 	}
-	}
 	return 0;
 }