USB: Implement support for "split" endian OHCI

This patch separates support for big endian MMIO register access
and big endian descriptors in order to support the Toshiba SCC
implementation which has big endian registers but little endian
in-memory descriptors.

It simplifies the access functions a bit in ohci.h while at it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>