ide-disk: add ide_do_setfeature() helper

Add ide_do_setfeature() helper and convert set_{wcache,acoustic}()
to use it.

There should be no functional changes caused by this patch.

Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>