nvme: refactor set_queue_count

Split out a helper that just issues the Set Features and interprets the
result which can go to common code, and document why we are ignoring
non-timeout error returns in the PCIe driver.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>