usb: ehci: fully align interrupt QHs/QTDs

These data structures are passed to cache-flushing routines, and hence
must be conform to both the USB the cache-flusing alignment requirements.
That means aligning to USB_DMA_MINALIGN. This is important on systems
where cache lines are >32 bytes.

Signed-off-by: Stephen Warren <swarren@nvidia.com>