block: move REQ_TYPE_ATA_TASKFILE and REQ_TYPE_ATA_PC to ide.h

These values are only used by the IDE driver, so move them into it
by allowing drivers to take cmd_type values after the first private
one.  Note that we have to turn cmd_type into a plain unsigned integer
so that gcc doesn't complain about mismatching enum types.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>