USB: EHCI: create a "periodic schedule info" struct

This patch begins the process of unifying the scheduling parameters
that ehci-hcd uses for interrupt and isochronous transfers.  It
creates an ehci_per_sched structure, which will be stored in both
ehci_qh and ehci_iso_stream structures, and will contain the common
scheduling information needed for both.

Initially we merely create the new structure and move some existing
fields into it.  Later patches will add more fields and utilize these
structures in improved scheduling algorithms.

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