xhci: Make xHCI driver endian-safe

Hi Sarah,

Reposting the whole set, since there're 4 instead of 5 having dropped the 'add debug' patch in the middle and I thought differing 'vN a/b' subjects may get confusing otherwise. (Two of the patches haven't changed. :/ )

These remove the scratty temp variables and comment the return value of xhci_handle_event().

Cheers,

Matt