nfp: devlink: allow flashing the device via devlink

Devlink now allows updating device flash.  Implement this
callback.

Compared to ethtool update we no longer have to release
the networking locks - devlink doesn't take them.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>