powerpc/nvram: Fix endian issue when using the partition length

When reading partitions, the length has to be translated from
big endian to the endian order of the host. Similarly, when writing
partitions, the length needs to be in big endian order.

The userspace tool 'nvram' needs a similar fix as it is reading
and writing partitions through /dev/nram :

    http://sourceforge.net/p/powerpc-utils/mailman/message/31571277/

Signed-off-by: Cedric Le Goater <clg@fr.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>