libata: support the ata host which implements a queue depth less than 32

The sata on fsl mpc8315e is broken after the commit 8a4aeec8
("libata/ahci: accommodate tag ordered controllers"). The reason is
that the ata controller on this SoC only implement a queue depth of
16. When issuing the commands in tag order, all the commands in tag
16 ~ 31 are mapped to tag 0 unconditionally and then causes the sata
malfunction. It makes no senses to use a 32 queue in software while
the hardware has less queue depth. So consider the queue depth
implemented by the hardware when requesting a command tag.

Fixes: 8a4aeec8

 ("libata/ahci: accommodate tag ordered controllers")
Cc: stable@vger.kernel.org
Signed-off-by: Kevin Hao <haokexin@gmail.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>