ide: add ide_tf_load() helper

Based on the earlier work by Tejun Heo.

* Add 'tf_flags' field (for taskfile flags) to ide_task_t.

* Add IDE_TFLAG_LBA48 taskfile flag for LBA48 taskfiles.

* Add IDE_TFLAG_NO_SELECT_MASK taskfile flag for __ide_do_rw_disk()
  which doesn't use SELECT_MASK() (looks like a bug but it requires
  some more investigation).

* Split off ide_tf_load() helper from do_rw_taskfile().

* Convert __ide_do_rw_disk() to use ide_tf_load().

There should be no functionality changes caused by this patch.

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