USB: fix scheduling of Iso URBs in uhci-hcd

This patch (as1003) changes uhci-hcd to treat the URB_ISO_ASAP flag
the same as other host controller drivers, namely, to schedule an Iso
URB for the first available time slot that hasn't already expired.
URBs in which the flag isn't set will be scheduled for the first slot
following the last URB, even if it has expired.

This fixes a problem reported by Martin Bachem.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>