xhci: Make xHCI driver endian-safe

This patch changes the struct members defining access to xHCI device-visible
memory to use __le32/__le64 where appropriate, and then adds swaps where
required.  Checked with sparse that all accesses are correct.

MMIO accesses use readl/writel so already are performed LE, but prototypes
now reflect this with __le*.

There were a couple of (debug) instances of DMA pointers being truncated to
32bits which have been fixed too.

Signed-off-by: Matt Evans <matt@ozlabs.org>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>