libata: move 'struct ata_taskfile' and friends from ata.h to libata.h

Move 'struct ata_taskfile', ata_prot_flags() and their friends from
<linux/ata.h> to <linux/libata.h>. They were misplaced from the beginning,  as
<linux/ata.h> should cover ATA/ATAPI and related standards only -- to which the
aforementioned structure and function have only remote relation.

I would have moved 'enum ata_tf_protocols' closely related to 'struct
ata_taskfile' but it unfortunately gets used by 'drivers/ide/ide-ioctls.c'...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Tejun Heo <tj@kernel.org>