nvme: fix the definition of the doorbell buffer config support bit

NVMe 1.3 specification defines the Optional Admin Command Support feature
flags, bit 8 set to '1' then the controller supports the Doorbell Buffer
Config command. Bit 7 is used for Virtualization Mangement command.

Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Fixes: f9f38e33 ("nvme: improve performance for virtual NVMe devices")
Cc: stable@vger.kernel.org