usb: return error code instead of 0 in the enqueue function.

if the enqueue function returns -ESHUTDOWN or -ENOMEM then
we return 0 instead of an error. This leads to a timeout and
then to a dequeue request of an not enqueued urb.

Signed-off-by: Sebastian Siewior <sebastian@breakpoint.cc>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>