mlxsw: Move firmware flash implementation to devlink

Benefit from the devlink flash update implementation and ethtool
fallback to it and move firmware flash implementation there.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>