nvme-pci: skip nvme_write_sq_db on empty rqlist

nvme_submit_cmds() should check the rqlist before calling
nvme_write_sq_db(); if the list is empty, it must return immediately.

Fixes: beadf008 ("nvme-pci: reverse request order in nvme_queue_rqs")
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>