ide: Fix build break caused by "ide: remove ideprobe_init()"

Fix build break of powerpc holly_defconfig:

In file included from arch/powerpc/platforms/embedded6xx/holly.c:24:
include/linux/ide.h:1206: error: 'CONFIG_IDE_MAX_HWIFS' undeclared here (not in a function)

There's no need to have a sized array in the prototype, might as well
turn it into a pointer.

It could probably be argued that large parts of the include file can be
covered under #ifdef CONFIG_IDE, but that's a larger undertaking.

Signed-off-by: Olof Johansson <olof@lixom.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>