nvme: copy MTFA field from identify controller

We use the controller's reported maximum firmware activation time as our
timeout before resetting a controller for a failed activation notice,
but this value was never being read so we could only use the default
timeout. Copy the Identify Controller MTFA field to the corresponding
nvme_ctrl's mtfa field.

Fixes: b6dccf7f (“nvme: add support for FW activation without reset”).
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Minwoo Im <minwoo.im@samsung.com>
Signed-off-by: Laine Walker-Avina <laine.walker-avina@intel.com>
[changelog, fix endian]
Signed-off-by: Keith Busch <keith.busch@intel.com>