xhci: Fix some DMA host endian bugs

xhci: Fix some DMA host endian bugs

The xhci device does correct endian switches on the results of some DMAs
but not all.  In particular, there are many DMAs of what are essentially
arrays of 32-bit integers which never get byteswapped.  This causes them
to be interpreted incorrectly on big-endian hosts, since (as per the xhci
spec) these arrays are always little-endian in guest memory.

This patch adds some helper functions to fix these bugs.  This may not be
all the endian bugs in the xhci code, but it's certainly some of them and
the Linux guest xhci driver certainly gets further with these fixes.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>