remove <linux/ata.h> include from <linux/hdreg.h>

All <linux/hdreg.h> users that need <linux/ata.h> have been fixed
to include it directly.

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