ata: pass queued command to ->sff_data_xfer method

For Atari Falcon PATA support we need to check the current command
in its ->sff_data_xfer method.  Update core code and all users
accordingly.

There should be no functional changes caused by this patch.

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Tejun Heo <tj@kernel.org>