nvme-pci: allow use of cmb on v1.4 controllers

[ Upstream commit 20d3bb92 ]

Since NVMe v1.4 the Controller Memory Buffer must be explicitly enabled
by the host.

Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
[hch: avoid a local variable and add a comment]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>