usb: add plumbing for updating interrupt endpoint interval state

xHCI caches device and endpoint data after the interface is configured,
so an explicit command needs to be issued for any device driver wanting
to alter the polling interval of an endpoint.

Add usb_fixup_endpoint() to allow drivers to do this. The fixup must be
called after calculating endpoint bandwidth requirements but before any
URBs are submitted.

If polling intervals are shortened, any bandwidth reservations are no
longer valid but in practice polling intervals are only ever relaxed.

Limit the scope to interrupt transfers for now.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.org>