ide: include <linux/hdreg.h> only when needed

* Include <linux/ata.h> directly in <linux/ide.h>
  instead of through <linux/hdreg.h>.

* Include <linux/hdreg.h> only when needed.

Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>