nvme-pci: split nvme_dev_add

Split nvme_dev_add into a helper to actually allocate the tag set, and
one that just update the number of queues.  Add a local variable for
the tag_set to clean up the code a bit.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>