ide: add ide_do_test_unit_ready() helper

* Add ide_do_test_unit_ready() helper and convert ide-{floppy,tape}.c
  to use it.

* Remove no longer used idetape_create_test_unit_ready_cmd().

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>