alim15x3: Remove historical hacks, re-enable init_hwif for PowerPC

Some time ago we had to disable init_hwif callback for PowerPC builds.
That was because of a historical IRQ overwrite in the driver, which
was causing IDE malfunction on the MPC8610HPCD PowerPC boards.

It's unclear whether this overwrite is still useful, but it is proven
to cause a bit of harm, and today some PowerPC targets (Xilinx ML510,
as reported by Roderick Colenbrander) need the init_hwif, so we have
to re-enable it and remove the overwrite.

Reported-by: Roderick Colenbrander <thunderbird2k@gmail.com>
Suggested-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>