USB: Fix: Don't skip endpoint descriptors with maxpacket=0

USB: Fix: Don't skip endpoint descriptors with maxpacket=0

It turns out that even though endpoints with a maxpacket length of 0
aren't useful for data transfer, the descriptors do serve other
purposes.  In particular, skipping them will also skip over other
class-specific descriptors for classes such as UVC.  This unexpected
side effect has caused some UVC cameras to stop working.

In addition, the USB spec requires that when isochronous endpoint
descriptors are present in an interface's altsetting 0 (which is true
on some devices), the maxpacket size _must_ be set to 0.  Warning
about such things seems like a bad idea.

This patch updates an earlier commit which would log a warning and
skip these endpoint descriptors.  Now we only log a warning, and we
don't even do that for isochronous endpoints in altsetting 0.

We don't need to worry about preventing endpoints with maxpacket = 0
from ever being used for data transfers; usb_submit_urb() already
checks for this.

Reported-and-tested-by: Roger Whittaker <Roger.Whittaker@suse.com>
Fixes: d482c7bb0541 ("USB: Skip endpoints with 0 maxpacket length")
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://marc.info/?l=linux-usb&m=157790377329882&w=2
Link: https://lore.kernel.org/r/Pine.LNX.4.44L0.2001061040270.1514-100000@iolanthe.rowland.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>