USB: usbfs: struct async is a fixed size structure

The ISO descriptors are allocated separately in proc_submiturb for a fetch
from user mode, then tucked at the end of URB. This seems like a dead code.

Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>