Fix a regression that was introduced by commit
811c926c (USB: EHCI: fix HUB TT scheduling
issue with iso transfer).

We detect an error if next == start, but this means uframe 0 can't be allocated
anymore for iso transfer...
Reported-by: NSander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: NMatthieu CASTET <castet.matthieu@free.fr>
Acked-by: NAlan Stern <stern@rowland.harvard.edu>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>