nvme: temporary fix for Apple controller reset

Recent patches added basic support for the Apple NVMe controller but
still cause resets and data corruption on that particular controller
when a specific pattern of read/flush commands occurs. Limiting the
queue depth to 2 works around that issue.

This patch enforces that limit only for the Apple controller and is
considered a temporary fix until we find the root source of that
problem.

Signed-off-by: Stephan Günther <guenther@tum.de>
Signed-off-by: Maurice Leclaire <leclaire@in.tum.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>