ide: add ide_for_each_present_dev() iterator

* Add ide_for_each_present_dev() iterator and convert IDE code to use it.

* Do some drive-by CodingStyle fixups in ide-acpi.c while at it.

There should be no functional changes caused by this patch.

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