libata: fix spurious WARN_ON_ONCE() on port freeze

Commit 54c38444

 makes libata abort qcs
after the port is frozen.  This is necessary to guarantee that TF
registers are accessed after the DMA engine is shutdown after an
error.  However, this triggers WARN_ON_ONCE() check in
ata_qc_complete() spuriously.  Move WARN_ON_ONCE() downwards such that
failing commands while frozen doesn't trigger it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable <stable@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>