ide/powermac: Fix use of uninitialized pointer on media-bay

The current ide-pmac calls media_bay_set_ide_infos() with an
uninitialized "hwif" argument. The proper fix is to split the
allocation of the hwif from its registration in order to properly
setup the mediabay informations before registration.

Acked-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>