USB: UHCI: improve scheduling of interrupt URBs

This patch (as1140) adds a little intelligence to the interrupt-URB
scheduler in uhci-hcd.  Right now the scheduler is stupid; every URB
having the same period is assigned to the same slot.  Thus a large
group of period-N URBs can fill their slot and cause -ENOSPC errors
even when all the lower-period slots are empty.

With the patch, if an URB doesn't fit in its assigned slot then the
scheduler will try using lower-period slots.  This will provide
greater flexibility.  As an example, the driver will be able to handle
more than just three or four mice, which the current driver cannot.

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