nvme-pci: init shadow doorbell after each reset

The spec states:

  "The settings are not retained across a Controller Level Reset"

Therefore the driver must enable the shadow doorbell, after each reset.

This was caught while testing the nvme driver over upcoming nvme-mdev
device.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Minwoo Im <minwoo.im@samsung.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>