ide: make ide_transfer_stop idempotent

ide: make ide_transfer_stop idempotent

There is code checking s->end_transfer_func and it was not taught about
ide_transfer_cancel.  We can just use ide_transfer_stop because
s->end_transfer_func is only ever called in the DRQ phase.

ide_transfer_cancel can then be removed, since it would just be
calling ide_transfer_halt.

Signed-off-by: Paolo Bonzini <[email protected]>
Signed-off-by: John Snow <[email protected]>
Reviewed-by: John Snow <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-id: 20180606190955[email protected]
Signed-off-by: John Snow <[email protected]>