[POWERPC] iSeries: Simplify viocd initialisation

We don't need to keep a lump of dma coherent memory around for the life
of the module.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>