ide: add ide_queue_pc_head() helper

* Move REQ_IDETAPE_* enums to <linux/ide.h>.

* Add ide_queue_pc_head() and convert ide-{floppy,tape}.c to use it
  instead of ide*_queue_pc_head().

* Remove no longer used ide*_queue_pc_head().

There should be no functional changes caused by this patch.

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