ata/acard_ahci: remove unused variable n_elem

No one care the varible acard_ahci in func acard_ahci_qc_prep.
better to remove it.

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-ide@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>