UHCI: implement new semantics for URB_ISO_ASAP

This patch (as1613) updates the isochronous scheduling in uhci-hcd to
match the new semantics for URB_ISO_ASAP.  The amount of code
alteration is smaller than it looks because of a change in the
indentation level.

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