sata_dwc_460ex: Fix missing check in sata_dwc_isr

The return value of ata_qc_from_tag() is checked in the whole
kernel except for two calls in sata_dwc_isr(), which may lead
to null-pointer-dereference. Add return value checks to avoid
such case.

Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Signed-off-by: Jens Axboe <axboe@kernel.dk>