USB: Fix up URB error codes to reflect implementation.

Documentation/usb/error-codes.txt mentions that urb->status can be set to
-EXDEV, if the isochronous transfer was not fully completed.  However, in
practice, EHCI, UHCI, and OHCI all only set -EXDEV in the individual frame
status, never in the URB status.  Those host controller actually always
pass in a zero status to usb_hcd_giveback_urb, and rely on the core to set
the appropriate status value.

The xHCI driver ran into issues with the uvcvideo driver when it tried to
set -EXDEV in urb->status, because the driver refused to submit URBs, and
the userspace camera application's video froze.

Clean up the documentation to reflect the actual implementation.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>